For as long as there have been flashlights, there have been people wishing they could mount them to a weapon. If you've ever tried to use a hand held light in conjunction with a weapon it becomes instantly clear that doing so is a pain in the neck. It's awkward, uncomfortable, and I've never met anyone who found it "easy" to do. Dedicated weapon lights were invented to help make the process of using a light and a weapon at the same time easier.
Handguns
Today the two most popular options on the market are the Streamlight TLR-1 and the Surefire X series weapon lights….and both are very good units. The TLR-1 is by all accounts a durable, reliable, eminently useful weapon light option….and coming in at around 100 bucks it is probably the best value in a weapon light currently on the market. Insight recently released the Procyon weapon light which I've been testing for a while. So far it's also proven to be a very good light for a handgun.
The Surefire X-series weapon lights are my personal choice for a dedicated handgun mounted weapon light. I prefer the Surefire units because they are a bit more compact and have a mounting system that I like better than the TLR-1's screw based system. They also have much better tape switches available than the TLR-1 does. My second choice would be the Procyon.
With these weapon lights there are essentially two types of controls:

1. Rocker switches
A switch mounted directly to the back of the light unit that is activated by moving the switch up or down by a either the trigger finger or a finger on the weak hand.
The TLR-1, X series and the various M3 derivative lights all come with rocker switches as a standard option. For a number of people the standard rocker switch is just fine. Others, however, have trouble using them effectively. I have found that rocker switches on weapon lights just don't work well for me. My fingers are considerably shorter than the length of my palm making it difficult for me to reach out in front of the trigger guard on many handguns to use the rocker switch on a weapon light. On a handgun like a Glock I don't have much trouble with it, but on something like a Beretta 92 it's incredibly difficult for me. As a result I prefer tape switches for my handgun mounted lights

2. Tape switches
Pressure activated switches that are connected to the back of the light by a wire and that can generally be mounted on the grip of the weapon, allowing a squeeze to the grip of the weapon to activate the light.
Like everything else in the "tactical" world, tape switches on handguns bring their own unique challenges and issues to the table. The biggest is accidental light discharges.
As I stated earlier, light is a target indicator. You want to use a minimum amount of light when you are searching a structure or doing something similar because you don't want the bad guy to know where you are. Tape switches are easier to use on purpose than rocker switches, but the downside to them is that they are also easier to use by accident than rocker switches, which can lead to lighting yourself or your teammates (if you do the CQB thing for a living) by accident.
The consequences of this can range from being a minor inconvenience to alerting the enemy to your presence and getting yourself and/or team members killed. The consequences for the average citizen or law enforcement officer are typically going to be on the very low end of that spectrum, thus I would say that the higher risk of light AD's with tape switches is a good trade-off for those individuals (like myself) who find that rocker switches don't work well for them.
I stated earlier that I prefer the tape switches used by the Surefire X series lights and now I'll explain why: Many tape switches out there are held on to the grip of a handgun via an adhesive, or Velcro that is glued to the grip and the tape switch with an adhesive. These do not tend to be good long term mounting solutions, and they don't offer the freedom to take the light on and off of the weapon at will.
The Surefire X series lights, however, have the "DG" switch which is molded around a stiff metal insert that is specifically designed for mounting on a particular type of weapon. This means you can take the X series light (an X200B in the pictures) on and off of the weapon at will without worry. You also don't have to find ranger bands or similar tricks to hold the switch on the grip of the weapon for hard use.
Now all of that is great for those who have a weapon with an integrated accessory rail, but what if your weapon doesn't have one? Thankfully the various light companies have realized that there are lots of people out there who don't have rails on their handguns and have figured out various aftermarket solutions for this problem.

One such solution is the Surefire MR-11 mounting rail you see here.
The MR-11 attaches to the front of the trigger guard on the Beretta 92 without damaging the finish, and allows you to mount a Surefire X series weapon light.

Other manufacturers also offer similar adapters, but the Surefire add-on rails are the best I've seen from anyone.
They are available for a number of service weapons like the Sig P226, the 1911, and the H&K USP.
Okay…. so you now have a weapon light and you've got it mounted to your weapon...but how do you carry the bloody thing? There are a number of holster makers out there who offer carry options for weapon mounted lights.
Safariland offers versions of their excellent law-enforcement holsters (like the 6004) to accommodate handguns with weapon lights, and so do other makers like Com-Tac, Blade-Tech, etc.
Unfortunately when it comes to concealed carry of a handgun with a weapon light your options are significantly narrowed. Most of the previously mentioned holsters are belt holsters that require some sort of covering garment. If you want to carry IWB it's decidedly harder to find a holster to accommodate that.

Thankfully Raven Concealment has come up with a great system that allows you to take one of their belt holsters and convert it into an IWB if you choose to do so.
As you can see in the photograph the Raven Concealment system uses screws to hold on the belt loops.
You can remove the belt loops and replace them with rubber IWB loops on the front of the holster, J hooks, or even tuckable loops.

Their standard belt configuration is also the most concealable holster for a mounted handgun on a light that I've found.
While a mounted light does add some bulk to the weapon, theRaven Concealment holsters handle it better than any other design I've tried.
Lots of very experienced people are buying gear fromRaven Concealment and consider them the best kydex maker around right now.
I'm very pleased with the gear I have from them.
Handguns
Today the two most popular options on the market are the Streamlight TLR-1 and the Surefire X series weapon lights….and both are very good units. The TLR-1 is by all accounts a durable, reliable, eminently useful weapon light option….and coming in at around 100 bucks it is probably the best value in a weapon light currently on the market. Insight recently released the Procyon weapon light which I've been testing for a while. So far it's also proven to be a very good light for a handgun.
The Surefire X-series weapon lights are my personal choice for a dedicated handgun mounted weapon light. I prefer the Surefire units because they are a bit more compact and have a mounting system that I like better than the TLR-1's screw based system. They also have much better tape switches available than the TLR-1 does. My second choice would be the Procyon.
With these weapon lights there are essentially two types of controls:

1. Rocker switches
A switch mounted directly to the back of the light unit that is activated by moving the switch up or down by a either the trigger finger or a finger on the weak hand.
The TLR-1, X series and the various M3 derivative lights all come with rocker switches as a standard option. For a number of people the standard rocker switch is just fine. Others, however, have trouble using them effectively. I have found that rocker switches on weapon lights just don't work well for me. My fingers are considerably shorter than the length of my palm making it difficult for me to reach out in front of the trigger guard on many handguns to use the rocker switch on a weapon light. On a handgun like a Glock I don't have much trouble with it, but on something like a Beretta 92 it's incredibly difficult for me. As a result I prefer tape switches for my handgun mounted lights

2. Tape switches
Pressure activated switches that are connected to the back of the light by a wire and that can generally be mounted on the grip of the weapon, allowing a squeeze to the grip of the weapon to activate the light.
Like everything else in the "tactical" world, tape switches on handguns bring their own unique challenges and issues to the table. The biggest is accidental light discharges.
As I stated earlier, light is a target indicator. You want to use a minimum amount of light when you are searching a structure or doing something similar because you don't want the bad guy to know where you are. Tape switches are easier to use on purpose than rocker switches, but the downside to them is that they are also easier to use by accident than rocker switches, which can lead to lighting yourself or your teammates (if you do the CQB thing for a living) by accident.
The consequences of this can range from being a minor inconvenience to alerting the enemy to your presence and getting yourself and/or team members killed. The consequences for the average citizen or law enforcement officer are typically going to be on the very low end of that spectrum, thus I would say that the higher risk of light AD's with tape switches is a good trade-off for those individuals (like myself) who find that rocker switches don't work well for them.
I stated earlier that I prefer the tape switches used by the Surefire X series lights and now I'll explain why: Many tape switches out there are held on to the grip of a handgun via an adhesive, or Velcro that is glued to the grip and the tape switch with an adhesive. These do not tend to be good long term mounting solutions, and they don't offer the freedom to take the light on and off of the weapon at will.
The Surefire X series lights, however, have the "DG" switch which is molded around a stiff metal insert that is specifically designed for mounting on a particular type of weapon. This means you can take the X series light (an X200B in the pictures) on and off of the weapon at will without worry. You also don't have to find ranger bands or similar tricks to hold the switch on the grip of the weapon for hard use.
Now all of that is great for those who have a weapon with an integrated accessory rail, but what if your weapon doesn't have one? Thankfully the various light companies have realized that there are lots of people out there who don't have rails on their handguns and have figured out various aftermarket solutions for this problem.

One such solution is the Surefire MR-11 mounting rail you see here.
The MR-11 attaches to the front of the trigger guard on the Beretta 92 without damaging the finish, and allows you to mount a Surefire X series weapon light.

Other manufacturers also offer similar adapters, but the Surefire add-on rails are the best I've seen from anyone.
They are available for a number of service weapons like the Sig P226, the 1911, and the H&K USP.
Okay…. so you now have a weapon light and you've got it mounted to your weapon...but how do you carry the bloody thing? There are a number of holster makers out there who offer carry options for weapon mounted lights.
Safariland offers versions of their excellent law-enforcement holsters (like the 6004) to accommodate handguns with weapon lights, and so do other makers like Com-Tac, Blade-Tech, etc.
Unfortunately when it comes to concealed carry of a handgun with a weapon light your options are significantly narrowed. Most of the previously mentioned holsters are belt holsters that require some sort of covering garment. If you want to carry IWB it's decidedly harder to find a holster to accommodate that.

Thankfully Raven Concealment has come up with a great system that allows you to take one of their belt holsters and convert it into an IWB if you choose to do so.
As you can see in the photograph the Raven Concealment system uses screws to hold on the belt loops.
You can remove the belt loops and replace them with rubber IWB loops on the front of the holster, J hooks, or even tuckable loops.

Their standard belt configuration is also the most concealable holster for a mounted handgun on a light that I've found.
While a mounted light does add some bulk to the weapon, theRaven Concealment holsters handle it better than any other design I've tried.
Lots of very experienced people are buying gear fromRaven Concealment and consider them the best kydex maker around right now.
I'm very pleased with the gear I have from them.