Amazon has a bunch of free stuff up right now.
Last night I finished downloading and am listening to right now Deep Elm Sampler No. 8 "Bonfire of Trust." Sixteen tracks by a bunch of different alt rock groups who I've never heard of, but it's okay stuff. The first piece by a group called Ride Your Bike gave me a brief Camper Van Beethoven flashback. Another group called 500 Miles to Memphis has definite country/bluegrass overtones.
Follow this link to get a list of free holiday mp3s. If you're like me, most of them won't interest you, but there are some notable tracks in the mix: Mannheim Steamroller, Vince Guaraldi, Bela Fleck, Brian Setzer.
And finally, there's another jazz sampler available, simply titled Amazon Jazz Sampler. I can't comment on it, since I haven't downloaded it yet, but there it is. It looks to be a bunch of newer stuff. No classics on this one.