If you're currently riding your bike at night, you've hopefully already got some sort of light alerting cars of your presence, but Make's guide to building a mini bike light from a garden hose adapter is too inexpensive and fun to pass up. It also requires just a few LCDs, some wiring, a switch, and some electronics know-how (Make has some great introductory guides if you're interested in this kind of tinkering), and when you're done you've got a very cool, bright little LED for your bike.
How-To Tuesday: Mini bike light [Make]