You could buy one of ASICMiner's USB ASIC devices. A bunch have hit ebay:

http://www.ebay.com/sch/i.html?_odkw=usb+asic&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0&_nkw=usb+bitcoin+asic&_sacat=0

At current difficulty, I calc that one would generate about 0.01BTC / day. Obviously that's going to go down significantly in the upcoming months as more ASIC hardware hits the network.


Or something like bitinstant.com ?