Asterisk:
Asterisk is a software-based Public Branch Exchange (PBX) that does about anything you can think to do with a phone. And since the software is Open Source, it is very reasonable to implement. This project probably takes up most of my free time, if for no other reason than its flexibility makes for a large learning curve.

The Barmonkey:
The Barmonkey is a nifty little device that I read about first on Slashdot.org. It is a computer operated, automated drink mixing machine. Once I get some time and some more parts, I hope to build one of my own. :)