While doing some research for GetLoFi I came across a spectacular page full of DIY guitar effect pedals. The www.moosapotamus.net/ contains not only neat pictures and sounds but also schematics and inside shots that will make you drool all over the keyboard. Everything seems fairly simple with some amazing sounds. The Tri-negistor was my favorite, all transistor device that uses negistors and negative resistance to oscillate like crazy. Each pedal is very well documented and referenced. There are also a few mentions of the StompBox Cookbook with pictures and schematics.
[Found through inverseroom.com]

*Updated 03/09/2010 * Fixed the URL; schematics seem to still be up