A streaming music site, like Last.fm or Pandora, but with all Free Culture music.
As a consequence, a bunch of nice features are possible that would otherwise give the lawyers heart attacks. For example, there are no ads and no DRM or anything, so the in-browser player is super lightweight. You can skip as much as you want, seek around within a track, and replay previous tracks. And, unlike the last time I tried the site a year or two ago, the music is actually good.
Libre.fm
Libre.fm: