I went for the Teensy 2.0 board. It's sort of an arduino clone, but cheaper and with more memory. The reasons I went for something other than original Arduino boards is price - only $16 (frugal bastard) and the small size like Arduino Nano - not available at the time I bought in. For a total of $31 I got the Teensy 2.0 with header pins, breadboard, and the tutorial kit.
Teensy USB Development Board
A good reason for choosing Arduino is all the expansion shields. Just stak'em up, code a little and you've got a working prototype.
If money is tight, check out
iStore, Make Innovation Easier - they are quite cheap and have a lot of parts, shields and some arduino "clones" (They aren't technically clones as the Arduino design is open source)
There's also a lot of parts to be found on eBay.