Along with Qtrax, www.Spiralfrog.com is another windows only compatible site. The reason is because the songs are encoded with protected-WMA, which is not compatible with Ipod's or Zune's. Both of these sites use advertisements to pay for the music. I think that Qtrax makes you listen to a few ads before you listen to a song, and Spiral Frog just has ads all around their site. So if you have a windows device, and don't mind advertisements, then these sites are just right for you. (Personally I use a mac and an Ipod 5th gen, so I'm not able to use these sites, instead I use Itunes) If you have any questions or comments PLEASE feel free to ask or say them.
Update:I have just found out that Spiral Frog has been shut down. For good? I'm not sure, but ad supported sites are not easy to keep up.