It’s really easy to use the free VUSB code to add USB for an 8bit AVR. It will be a good way to add stuff to the Pi. The tiny85 (which I get for .75 ea) can even do so without needing a crystal as the adjustable oscillator gets within 1% at 16.5mhz ... so all you need is the software and a couple of 3.6v zeners and you have a 16mips AVR hooked to the Pi via USB.
A one dollar tiny88 with a 7 cent 12mhz xtal is also good as you then have 28 IO lines with the tiny TQFP package. That’s a lot of added goodies for a total cost of less than 2 bucks. I’m going to get some small USB boards made up at iteadstudio.com for the tiny88.
You are not supposed to panelize with the 9.90 for 10 boards deal at itead but I did it before and just cut the boards in two myself ;-) total cost was about .70 each for the resultant 20 tiny 2-layer boards and that included the shipping cost.
/johnny