Blip.fm

Take one part of TheSixtyone (levelling, badges, user recommendation, community), take one part of Twitter (hashmarks, short description), one part of Grooveshark/Hypemachine (adding music from blogs), one dash of Last.fm (neighborhood/group-radio) and mix this with a very slick interface and you get Blip.fm.

What exactly is a ‘blip’?
Blip'ng a song means to play the song combined with a short comment and|or some hashmarks (did I mention Twitter yet?). That blip will then show up in the playlist of your listeners (Twitter translation: followers) who then get to to play and possibly reblip that song with their remark|hashtags. The more they reblip your blip (I feel like I'm qouting the Smurfs here) the more status you get and the higher your exposure will get.

Listening Badges
Some of the badges you can get are easy (I got Pioneer within 10 minutes by adding some mp3-urls from a post on Das Klienicum), others will take longer since they involve levels.

Radio
You can create a radio based on genres/artists/#hashtags but having used it I'd have to say it was somewhat  tempestuous to say the least (at the time the expletive "heinous b*tch" was uttered a few times). It's probably better to first play a bunch of artists and add suggested other users to your favorite sources to get a more diversified playlist.