Piezo Speaker Elements
Add sound to your robot! Piezo speakers elements can be driven directly by a PIC output, and are nice and small.
These are two varieties I picked up at the Electronic Goldmine.
This is the one you want to get. It's small, flat form factor make it easy to stick anywhere on your robot. It produces a pretty decent sound for such a small device.
I've used the above element to play buzzes, rising/falling tones, "Happy Birthday" (built into the Picaxe) and the musical notes used to signal the aliens in Close Encounters of the Third Kind.
Stay away from this one, unless you are going to provide your own amplifier circuit. I tried running this element from a Picaxe08 and a Picaxe40, and the sound is barely audible even with your ear right next to it.
Beware "piezo buzzers", which are really only designed to produce one tone when you apply voltage. Piezo elements with a wider frequency response like the ones above can reproduce tones generated by your PIC.