If your a music lover and also inclined to discover new (&old) music then last.fm is the place to be. I've signed up a couple of months back and have really been enjoying music surfing. Thats right! similar to channel surfing last.fm is about discovering music based on what you like and what others who are like you like. There is plenty of research in this field, some of which are headed by the folks at lastfm, but what it essentially means is a good trip down music lane :)
You can sign up for a free account and search based on artist or a tag such as pop, rock, jazz or female vocalist. In any case your search result becomes a virtual radio station which goes off on all sorts of tangents discovering similar artists or similar artists, tags and so on. When you hear a song you really like, you can express your pleasure by marking it as loved, or display your hate by banning it and you shall never hear it. You can also skip tracks, unlike web streaming radio but can not seek within the song, since it isn't meant to be "music on demand" sort of a service but more of a personalized radio like service.
The killer feature is scrobbling, which you can get by downloading and installing the Free/Open Source last.fm client on your computer or portable device such as an ipod touch/iphone or N800. What scrobbling essentially is tracking the music you play and uploading that information to the last.fm server against your profile to improve the kind of music you hear and to provide user listening patterns and charts.
The ipod touch/iphone client for those who've managed to jail break their device is simply awesome. I am so hooked on listening to music wirelessly while lying in bed and marking tracks I like and checking out lyrics, information on the artist using the integrated wiki lookup as well as checking artist tour dates and locations (not that I'll ever go but its fun). Unfortunately the desktop clients seems to lack some of these fine features integrated for the time being.
So if your music lover, go check it out. Its web 2.0 done for music where you can form your own little friend network and discover music together. if your really hooked on it, might even be worth dishing out a couple of extra bucks for the value added features the network provides.