A miniature tesla coil powered by a CRT flyback. It has a 450 turn 13.875cm tall secondary coil and a 7 3/4 turn primary, with a resonance frequency of 1.13MHz. The coil is capable of outputing several hundred thousand volts, and can produce arcs about 3cm in length. Not too impressive, but a good start.
A small 3d object scanner utilizing an Arduino Uno, a SHARP IR sensor and some stepper motors.