The Harrison Instruments site ain’t too much to look at, but they do have the biggest selection of Theremin kits I’ve ran across. A Theremin is one of the first electronic instruments, it uses radio frequencies to make noise as your hands get closer to it… Link.
Theremins galore!: DIY RCA Theremin Replica – Link.
HOW TO – Make a Theremin – Link.
What is a Theremin? – Link.
HOW TO – Make a Radio Shack optical Theremin – Link.
Optical theremin + soapdish – Link.