I began frequenting Purevolume during my sophomore year of high school. The site has evolved so much since that time; it's wonderful! Once you've accessed the site, click the "Top Downloads" tab on the homepage. Although not all songs located there will be free, the majority of them will be. Of course the free songs will be the most downloaded; who'd turn that down?
I've said it once and I'll say it again; I love thesixtyone.com! Being my personal favorite of all music sites, it's extremely innovative and quite fun! Although you have to sign up for an account to access the downloads, it's quick, painless, and sends you on your way to musical heaven immediately. Like Purevolume, not all downloads are free but a vast majority of them certainly are.
Insound.com offers one to two free downloads as a sample of each album they have for sale on their site. A great incentive to buy the full album, no? I've actually found free music from great bands on Insound, like Fruit Bats, Portugal The Man, and The Raveonettes. Win! With literally thousands of songs to choose from, Insound offers the most diverse selection of music. They even have a handy little search bar on the homepage to make your MP3 searching adventure a little less daunting and, dare I say it, intimidating.
So, there you have it! The complete list of my beloved music-hunting sites that I visit religiously. Although I use these sites the most often, I won't neglect to include a few other sites that regularly offer a great selection of free music if you're willing to search for it: http://www.pastemagazine.com/, http://www.fuse.tv/, http://www.urbanoutfitters.com/.