Arduino
The arduino is a great platform for prototyping. See more info at http://www.arduino.cc
Making your Arduino projects smaller:
When you are happy with you sketch and the way your arduino functions, you may want to consider scaling it down into a smaller package. This will allow you to fit it into to smaller packages.
Arduino Nano
The arduino nano is the first step down. It is basically just a smaller arduino. It keeps all the same pins, has a USB programmer and is just as easy to use as the UNO.
Arduino Pro-Mini
The arduino Pro-Mini is the next step down. It is a smaller and cleaner package, but you do sacrifice some core Arduino components like power regulation* and a USB connection.