Right now, on Amazon, you can look through 1,120 songs for free download.
Seriously -- go to this page, and you can sort through enough free songs to fill an iPod...well, my iPod, at least. Barsuk Records offers up tracks from the majority of their artists, from Death Cab For Cutie to Say Hi, and Def Jux does the same with tracks from Murs, Dizzee Rascal, and others.
I cannot recommend the eight song Alive Records sampler enough. It's available here. Alive (a subsidiary of classic New Wave label Bomp!) has been putting out some of the finest dirty blues influenced garage records of the past few years. If you like the White Stripes, and want something that sounds like them, but isn't another Jack White side project, this is for you.
Lastly, poke through the reggae section. In addition to new music from the likes of Lady Saw, you can find classics from Barrington Levy, Dennis Brown, and the Slickers.