Daytrotter is a website where you can download free live sessions of artists like Tegan and Sara, Vampire Weekend, Tokyo Police Club, Of Montreal, and tons more!
La Blogotheque's Take-Away Shows is a group of French guys that video impromptu concerts of people like Edward Sharpe and the Magnetic Zeros, Wilco, Phoenix, First Aid Kit, Sigur Ros, Fleet Foxes, sooooo many more!
NoiseTrade has various artists that offer their music for free. The catch is that you have to either email a friend or post a link on Facebook or Twitter. Or you can just pay for the music. I've gotten lots of Christmas music (that I'm already listening to!).
Let me know if there are other sites like this!
EDIT: I had to come back and share the iTunes free Elvis single. It's a remix of "Blue Suede Shoes" and it's awesome! It makes me dance...go get it while it's still free!