Last.fm is a online social music service where you can listen, get recommendations and find concerts near you, all based in your personal tastes.
Signing up is really easy and fast, then the next optional thing is to install their 'scrobbler'. This application runs in the background, recording your listening habits from iTunes, Winamp or Windows Media Player. This will build your Last.fm library, which is the basis of the service.
You library allows Last.fm to give you music recommendations and a neighborhood of other users with similar tastes. Your scrobbling also gives you various stats. You get a live list of your listening habits, and also your own charts.
Listening to music in Last.fm is a bit more limited. For playlists you create or recommendations, standard free users will usually only get thirty second previews of tracks. However, listening to to other user's radio stations, or that of your neighborhood doesn't have this restriction.
There are also Last.fm user groups, so you can find like-minded people, plus an excellent events section, where you can look for concerts in your area, and also see which ones other users are attending.
Last.fm is a great musical social network, with great recommendations, and one of the best ways to find out what's on anywhere in the world.