I bought some small tilt-sensors. basicly they where a small metal capsule with a ball inside. When tilted, the contact was switched on/off.
I thought this was smart, as the robot would then know for instance if a caterpillar-driven robot was on it´s way to flip backwards due to too steep terrain etc.
I also thought I could use them for balance, a robot would stick out a foot or something if it was too tilted to one side.
In practice I never made them do much good; The ball inside is constantly shaken by what ever else the robot is doing.
My founding is that they only work to detect if an elsewise static object is being lifted, moved or tilted.
My robots do not seam to be static enough :)