Rather than keep making new posts about random electronics shit, I'm making the Rolling Electronics Thread of Doom.
Item 1: Open-source (in every sense, including hardware) logic analyzer. http://dangerousprototypes.com/2010/04/27/logic-sniffer-quick-start-video/
Mine just came in the mail and it's pretty cool.
It's good for debugging hardware ("Is my chip outputting what I think it's outputting?") and for decoding other hardware ("I know this is SPI but how do I emulate it")