Radio Blog Club

Y'all should check out Radio Blog Club. It's got a lot of music (and of French music), all for free- stretching the limits of what can be called "online radio." You can search for an artist or a song, and then listen to it and a list of music that the site recommends for you, or add it to a "favorites" list and make your own playlists of songs you like.

(In sort of the same vein is Pandora, a site that will make a custom station for you based on a song or an artist that you like. It's really quite good, but has not much French music, so I don't listen to it all that often.)