My first arduino project, made primarily with parts from the linked Sparkfun wishlist.
I bought a Seeed Studio Protoshield to put together the final product.
Transistors are NPN, TIP31s I picked up at RadioShack (Yes, they still have project parts!). I don't know enough about this stuff to tell you much else.
(IMPORTANT EDIT: These Transistors can't handle the lights at full brightness. I need to find a replacement and edit accordingly. I wrote a work-around in the meantime: The MAX_SAFE_OUTPUT variable should be between 0 and 765. 765 is 255 on all three channels, full brightness. Anything less than that adjusts down the maximum output the code will allow, independent of any color cycling functions you choose.)