Seekasong provides free MP3 downloads. Simply type in either the artist or the track and Seekasong will provide a list of all the songs available. The best part about this site is that it offers songs from all over the world, not just exclusively in the United States. So if you're looking for that big hit you heard in Spain while on vacation, this site can help you find it and download it.
MP3-Center is a top-notch site if you looking for recent hits. It's nice for downloading songs by big-name artists such as Beyonce and Jay-Z, but not for songs by smaller, indie groups.
Emp3 World not only offers the top music of the week, but it also has a wide selection of music by smaller, indie groups. The site's search engine is extremely helpful---you will most likely be able to find the musician you're looking for with only a few clicks of the mouse.