Arduino
Arduino is an open source rapid prototyping platform based on easy to use hardware and software.
Arduinos are intended to be used by hobbyists, artists, designers, and anyone who’s interested in quickly and easily creating their own gadgets.
It is possible to connect almost anything to the Arduino board. I myself have successfully experimented with servos, ultrasound distance sensors, and the Wii Nunchuck, among others. What’s more, an Arduino can easily be connected up to a computer that can control the Arduino, and can receive data from it.
The Arduino can work with pretty much any computing platform, and the official IDE is available for Linux, Mac OS X, and even Windows.
You can get more information on Arduinos on the Arduino home page and on the Arduino Wikipedia article.
Ladyada.net also has a good guide to some of the basics of Arduino and electronics. (Thanks to Ivan Gitachu for the link)
For an idea of some of the things that you can do with Arduinos, go to Arduino Playground.
If you understand Finnish, you can also go to Sulautetut.fi