Many online music services allow you to give a thumbs up or thumbs down to songs that you enjoy or don't enjoy listening to. Jelli however leverages the thousands of votes logged by its users to build a better radio experience.
Jelli applies the experience of user-driven content — like user-drive news aggregators such as Reddit and Digg — to online radio. After you sign up for an account you can start listening to the Jelli stations — they'll be streamed to the media player of your choice via .m3u streaming — and as you're listening you can flag songs with the "Sucks" and "Rocks" buttons as well as look into the future of the play list via the sidebar in Jelli and vote on it accordingly.
Jelli is a free service but does require a basic registration process. Have your own favourite service for listening to new tunes? Let's hear about it in the comments.