Recently, I got a green laser from EBay for only $20.00 (usually these lasers cost $50-$80), to use with my robots. As you can see in the above picture, it's REALLY bright - the range is something like two miles.
The main advantage to a laser is that it puts a very bright light in one precise place. This can be used for things like making two robots align themselves to point towards each other (which I'm thinking of doing for my next project).
Anyway, I found that if I put the laser right on top of a light sensor and pointed it towards the ground (while the light sensor was level), the light sensor would only detect the beam if it reflected off an object a short distance away into the sensor. So I made an attachment for this and put it on Trax, my first successful rover that uses the new treads (using spring-loaded sprockets works great - thanks Brian!):
Below is a video of the robot detecting a wall... notice how the beam moves along the ground until coming across an object. Then it moves up the object as the robot gets closer to the object, until the beam reflects into the light sensor.
Although it was a fun idea, I have to say it's very impractical. The one theoretical advantage it has over the US sensor is that it can detect curved/slanted objects no problem - the reflection of the beam is just as bright no matter how curved or slanted the object is (except of course when the object is a mirror or another highly reflective material). However, usually when the robot comes at a wall from an angle it can't get close enough to the wall for the laser beam to get high enough. It's disadvantages include having a very small range of detection, and being susceptible to ambient light conditions and object textures. It might, however, be useful when combined with the US sensor to provide additional information about potential obstacles.
NOTE: Since lasers (especially green lasers) can harm your eyes if pointed directly at them, you should exercise caution when using them.