[keen101]

Sparkfun has a lot of cool sensors. So does parallax.

we've used:

CMU cam with moderate success
Parallax PING>>> ultrasonic sensor with success.
Limit switches
IR board
Potentiometers
1 wire temperature sensor (obtained from allelectronics)

all electronics
electronic goldmine
bg micro

are all places that sell surplus stuff, and usually have some cool sensors on the cheap.

oh, almost forgot..... last year we used line sensors as encoders for our wheels. We had to print out a circle that was split up into 8ths half dark half white, but it did work as wheel encoders to tell us relatively how far our robot had traveled. Feel free to use sensors in unexpected way's.