Made famous by Rolf Harris, although David Bowie used one on Space Oddity first. This thing makes a very distinctive noise. It is played by placing a metal stylus on a key (just a metal plate) to play the correct pitch. A cool thing you can do is slide the stylus up and down the keyboard for a glissando effect (not portamento). Be warned, the tuning on these things can drift mid-song. Worth borrowing one just to sample it.
Comments About the Sounds:
Well there is only one sound, but it is a classic.
(Thanks to Nick Clark for this info.)
and Markus Feuerstack for the pic