I've been playing around wiring my Pi to a low cost I2C module which has the following sensors:
- Accelerometer (x,y,z, although z is a bit flaky)
- Compass (x,y,z)
- Gyro (x,y,z)
- Air Pressure / altitude
At the moment I'm just logging air temperature and pressure - I'm sure I'll think of something useful to do with it.
The module can be had for about $US20 (I got mine from dx.com)