I built a range sensor for a group that would have to interface with an older electronic system (at a later date) and relay some information back to a PC. Came up with a smallish Cube shape that has an Ardweeny (to negotiate with the unknown hardware at a later date), a Sharp IR range sensor, Xbee + Xbee_explorer, and +5v external power.
Demonstration software, measures distance and flips from "Vacant" to "Occupied" at a set distance.
A very easy build, but the IR sensor was causing the Ardweeny to brown out when it pulsed. A cap on the power rails seemed to work perfectly. So far i'm impressed with the Ardweeny itself.. but i still don't have a good project for one yet.. anyone have an idea?