8 bit Shift Register IC - $3.99

Simple shift register IC. Clock in data and latch it to free up IO pins on your micro.  Basically a shift register will, in the end, let you expand on the digital outputs you have on your mictrocontroller.
Each one of these 74HC595s can act like 8 more digital outputs, and you can daisy chain them. So you could hook 8 of them up next to each other and have control of 64 outputs.

Price: $3.99