In high school I decided I was going to build my own analog synthesizer, I spent hours designing various oscillator circuits and patch panels. At school I took an Electronic Music class and got to use the Moog Model 15 to do various compositions. It wasn't until much later on Usenet after a big hoax had gone around about a 'new' Analog synth did I find out that Roger Arrick, I guy I knew from robotics work, was building synthesizers[1]. I got a smaller one from him (22 slots) about 10 years ago and have had so much fun with it.

[1] http://synthesizers.com -- the plural is important!