Learn how to set your atmega to work with various external crystals, program the fusebits, and build a fusebit doctor to revive your AVRs, in case you wrote the wrong settings .
The Sharp GP2Y10 dust sensor can detect dust in air using the reflected infrared light, including very fine smoke particles. See how to use it with a microcontroller.
A digital counter with a nice LCD display that supports both incrementing and decrementing. Good for various purposes, in this case as a display to a coil winding machine.