I've been using Last.fm for awhile now and (after a break) I've started using it again and placed a widget on this site for it (see sidebar). Last.fm is a social music sharing site that let's you "scrobble" songs (when you listen to music, the name of the song is sent to Last.fm and added to your music profile) and based on your listening trends you can discover artists you might like or connect with people who share your tastes. You can scrobble a few ways, including using the Last.fm client (which logs whatever you're playing in Winamp, iTunes, etc or let's you listen to personalized internet radio channels) or you can use a plug in such as iSproggler which logs plays from your iPod whenever you sync (I mostly do the latter). You can even listen to music through the site fairly easily.
Here's my profile page, and here's my music obsessed friend Sergey's. We have a music compatibility rating of "Medium" and, although I can deduce this by common sense, it's strangely comforting to be reassured by a cold, statistical algorithm.
After gathering enough data, it's exciting to look back and see your listening trends (just please ignore my fifth top artist at the time of this writing).
UPDATE (Mar 25, 08): I just found the MobileScrobbler app for the iPhone! Unfortunately you must have a jail-broken iPhone to install it until official iPhone apps are ready (this is the first time I've wanted to jail-brake my phone). Would be spectular to have whatever you listen to whereever you are be instantly logged (if you're a heavy iPod type user) or just be able to listen to Last.fm radio on the go.