Arduinos .. what can you do with them?
Arduino home page
The answer is LOTS....
Some of the things that I have done
They had an old 110v set of wired traffic lights and I refitted these to work wirelessly from a 12v battery.
Reaction game where the payer has to press the lit button.
Custom hardware and software solution to write to an EMUTAG for use ina Davinci XYZ printer.