Arduino is an open-source micro-controller platform using Atmel processors. It is a great way to learn about electronics and have a go. Also available and open is an integrated development environment (IDE), this allows you to program the Arduino via a standard computer. The language used it similar to Wiring.

Arduino IDE source code download

How to get the Arduino IDE working

Getting Arduino run properly on Puppy 5.28 and 5.3 and 5.31 thread
How to Install Arduino on Lupu-500 thread
Upgrade Arduino IDE from 21 to 22 (and beyond) thread

Arduino Linux installation help page
Arduino Tutorial on ladyada.net (recommended)