Miss Cellania's post on the Beer Bottle Orchestra is pretty hard to top - but this one comes close (well, it's geekier, anyhow): the Bubblegum Sequencer.
Links: Bubblegum Sequencer website | YouTube Video - Thanks Christophe!
What is the Bubblegum Sequencer?
The Bubblegum Sequencer is a physical step sequencer that lets you create drumloops by arranging colored balls on a tangible surface. It generates MIDI events and can be used as an input device to control audio hardware and software. Finally, people can't claim anymore that electronic music isn't handmade.
Here's how it works: A grid of holes, consisting of several rows with 16 holes each is the canvas. On it, you arrange colored gumballs. The 16 columns represent the 16th-notes in a measure. Each color is mapped to a specific sample.
Because the output is generated in the form of MIDI events, the Bubblegum Sequencer can be used to control any kind of audio hardware or software.