ASIC, and the only people with a working system on the free market today
http://launch.avalon-asics.com just raised their price to 88 BTC about $6000. They only take bitcoin and you have to wait 2 months for delivery!
The cheapest way to mine bitcoin by putting a graphics card in your PC.
You can get an Ati 5830 off ebay for £60/$90 and mine 260MH/s. (about BTC 0.8 pm)