You've got all kinds of choices for microcontrollers for your DIY projects. Adafruit's newest option, the Trinket, is a tiny, Arduino-powered board that fits on your fingertip.
The Adafruit Tinket is powered with an ATtiny85, and has the Arduino IDE so it can function essentially like a tiny Arduino board. That means you can upload small programs via USB, and it runs off very little power so it can keep going for a long time. It's a capable little board all things considered, and if you're in need of something very small for your projects it's worth a look.
Adafruit Trinket [Adafruit]