There are quite a few that fit on the rails. LaserMax and LaserLyte both also have rail mounted lasers in addition to the Rear Sight Laser and the Guide Rod Laser already mentioned, and both have information & pictures on their websites. The RSL & Guide Rod type both have the advantage of not interfering with the way the gun fits into whatever holster you already have though, while the rail mounted ones usually force you to buy a new holster.
Some people like their guide rod lasers, but some say they are fragile and break too much, and they are pretty expensive. Personally I think the RSL is the one that is easier to turn on & off with a quick thumb press while drawing the gun with one hand, which is also an advantage over all the rail mounted ones I've seen. It also seems pretty unbreakable even if you drop the gun, because it's made of steel.
My beef with Crimson Trace is that the grip lasers change the way the gun feels in my hand, which I don't like since my G20 grip is already big enough as is. Turning on the laser just by grip alone is also an issue if you wanted to ever draw the gun without turning on the laser, but not everyone is as bothered by that as I am.
I misread thought you said you had rails. In that case go to a store and look at both the RSL and the Guide Rod types. I think the RSL is more durable and drop resistant (I have one) and it's adjustable, while the guide rod lasers are not, and claim not to need adjustment even though every other laser in the world does. But your opinion of the sight picture through the RSL (when using the sights) will also be an issue. I like it, (to me it's the same as stock) but some people don't for some reason. You really have to hold a gun with one on it to find out what you think. They are a lot cheaper than guide rod lasers too.
Last edited by Brian Lee; 09-22-2010 at 15:53..