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Wheeled
One of the most important things after building a robot is making sure it doesn't suffer sudden deacceration syndrome (falling of an edge). So I have come up with the edge detector. A rather simple device that detectes edges. My idea is a metal ring with some of the surgical grade wire that you can get at solarbotics down the middle. The wire is good because its small, it bends easily and, doesn't stay bent. The way it works is the robot (wheeled or snake) goes forward the wire and ring arranged vertically so that the ring hovers parallel above the ground with the wire running straight down through it touching the ground. So the wire drags across the floor so that the wire is alwalys touching the ring. When the detector goes over the edge the wire snaps back to the center where its not touching the ring, when the robot backs up the sensor is rest. Requires that the robot is level and going over level ground (level except for the edge), or you could have a longer wire, but that creates extra drag. You can increase the sensitivity of the detector by lowering the ring, making ti smaller in diameter, this allows you to make the wire shorter.
Walkers
One way is to have an IR sensor on an extended pole on the front of the robot. The sensor would need very limited range so the sensor wouldn't recieve the reflected signal. This type of sensor would only work on fairly flat surfaces and it would need lots of fine tunning, just imagine trying to carfully turn a small turnpot while a walker is squirming wildly. Another idea is to have feelers like a lobster. The stalk could have two covered wires, at the end a mm a two would be exposed. The exposed area would be seperated by a plastic peice, or a bit of a tooth pick and it is then held together by heatshrink tubing. The idea is two small motors would lift and slam the end of the stalks against the ground, the wires touch for a split second and seperate again, if the ends don't make contact it backs up. Another idea is to two strips of thin and not too wide pieces of metal one or both is almost completely covered by heatshrink, then both peices are covered in heatshrink. Again the end is seperated by something small and round at the base of the stalk before it splits. Only the bottom peice needs to be bent downward, the feelers only needs to have a little bit exposed, and the ends should be covered so nothing ruins the sensors. This second idea is more stable and won't be twisted around or bent so the leads touch permanetly.