These music blogs have downloadable MP3s and, in my opinion, some quality music and commentary. These are the ones I habitually visit and collect music from. If you're looking for some new (often old but unfamiliar) free music, you might consider checking some of these out. Of course I'm looking for more music blogs, so maybe some of you have some to share.
The Rising Storm
http://therisingstorm.net/I love this site. Bunch of psychedelic folk, country rock, stuff like that. Great place to discover some 60's and 70's music you missed out on the first time (if you were alive). Note that all songs are downloadable with Real Player down-loader.
Aquarium Drunkard
http://www.aquariumdrunkard.com/All kinds of eclectic stuff. Includes mix tapes. Great stuff and frequently updated.
Cover Freak
http://coverfreak.com/This is ALL covers of other songs. Interesting site with commentary. The MP3 links disappear after a month or so, thus you have to download while it's fresh.
Forkast
http://pitchfork.com/forkcast/Newer more popular but alternative sorts of stuff.
Iron Leg
http://ironleg.wordpress.com/More of the older goodies. There are lots of "mix tapes" one can conveniently download.
Soul Sides
http://soul-sides.com/Soul for those of us who ignored this cool genre. There are tons of mix-tapes one can download as zipped files.
Prog Archives
http://www.progarchives.com/Encyclopedic site devoted to progressive rock and all its offshoots and sub-genres. Thousands of downloadable songs. Wish there were more sites like this for other broad genres.
WFMU Blog
http://blog.wfmu.org/freeform/mp3s/index.htmlThis is the best local non-commercial radio station in NYC, where I used to listen and download countless shows. Unfortunately the site works like shite in China and I can't download most the new material.