Last week Google introduced their Patent Search site, which searches patents going back hundreds of years. What could be more fun? Here is Bob Moog's patent for the Moog 'ladder' Filter, filed in 1966 and granted in 1969. This seems to be John M Chowning's patent for FM synthesis, which earned $20m for Stanford when Yamaha licensed it for the DX7. Here is Leon Theremin's 1925 patent for the Theremin. Here is Leo Fender's pickup patent from 1944 (here is Les Paul's). Here is the Synthaxe, here is the Fender
UPDATE: Rather than randomly putting words into Google, Don Tillman has actually researched this stuff. Here are his surveys of patents from: Moog, ARP and Mellotron/Birotron.
UPDATE 2: Casionova claims to have found the patent for the Demo Button, although I'm sure they had them before 1986. He also found this super-awesome Casio electric harmonica ("a main body having a plurality of ducts").
UPDATE 3: Eddie has found this 'Electronic Percussion Musical Instrument' designed by Florian Schneider and Ralf Hutter from Kraftwerk in 1975 (it's only an 'ornamental design' though).