Last.fm is a music discovery and listening service. It's got a wide range of tracks users can listen to and share with others. It also has a recommendation engine that will give you a list of artists it thinks you'll like, based your personal favorites. Users can make their own playlists and even share them with friends using the service's widget-making tool or Facebook application.
One of its more helpful services for music junkies is Scrobbling, which tracks the music you're listening to on your computer and sends the information to Last.fm. From there, everyone can look at what you've been listening to, how often you've listened to it, and give it a go themselves. Users also can have their own blogs where they can write about and reference music they're listening to, or just write about whatever they want.
Note: Last.fm is a part of CBS Interactive, which also publishes CNET News.