Here's a cool project for anyone who bikes after sundown: Turn an IKEA lamp into a solar-powered bike light, complete with extra-bright LEDs and light animations.
Instructables user bergerab posted the full instructions for this IKEA hack. The light has a 3-hour battery life and can be mounted to your backpack or your bike.
Besides the lamp, you'll need an attiny85 or attiny45 microcontroller, the LEDs, and a few other small parts. Bergerab provides the schematics and Arduino code for controlling the LED animations and modes.
Once done, you should have a bright, unique bike lamp so you can ride safely after dark.
IKEA Hack: Solar Powered Bike Light [Instructables]