PiTeR useful links - Guzunty/Pi GitHub Wiki
There were a couple of really useful links I found when designing PiTeRs balancing algorithm
A beginners guide to PID - This one was great for eliminating glitches from the PID algorithm.
KAS Bot - I started from this code but what I ended up with was a long way from where I started.
Balancing robot for dummies - This is a working link to the original balancing robot discussions in the Arduino forums. A lot of interesting thoughts in here.
Balanduino - I looked at this code, but while its an excellent project, I didn't use any of it because my hardware configuration was too different. It would be a good place to start if you're looking for a kit rather than starting from scratch like I did.