Last.fm's signup page is one of the best I've ever seen.  It's elegant, user-friendly, and does a fantastic job of keeping the user aware of what's going on. Rather than splitting the signup process into several steps across multiple pages or combining them all into one massive form,  it splits it into three steps but keeps everything on one page. As you go through the signup process the steps you've completed become greyed out.  It also uses just the right amount of AJAX to aid in the signup process. Kudos to the designers/developers that put this together. It'd be interesting to see if this process would work for an e-commerce checkout page.

Last.fm's Three Column Layout

As far as Last.fm's actual service goes, I've just gotten started with it so I can't vouch for how good it is.  It did win first place in the music section of our Web 2.0 Awards, however.