Right now, the easiest way to get in small is the Bitcoin Erupter. It is not at all economically viable to mine with a CPU/GPU anymore, even if you were to get your power for free.
They cost about .17 BTC each (about $20), or you can get discounts on orders of 5, 10 or 50. (I get mine at wtcr.ca)
They run off a power USB, and most people jsut run them in a hub like this:
Some people get creative with it, and do it like this though, as the devices do run a bit hot.
