I got a SAM3X Teensy 3.1 since this thread started and see another affordable option. The thing will overclock to 96MHz and loves to run Arduino code. My UNO cost more back when I started Arduino.
However... Teensy 3.1 needs extra careful soldering and the points are not all easy to get to, I'll be looking to pick up a Due (with more RAM, bigger than a thumbnail board with headers and stereo audio but over twice the price) for actual experimenting with.