Basically, anyone can stream any song available three times. No need to register/log-in, as a cookie on your computer tracks the plays (so, three plays at home, three plays at work, three more plays when you delete the last.fm cookie). After the three plays, you can still hear the song just like you could before on last.fm, by including it on a playlist for a customized radio station (the songs play in a random order, so no on-demand satisfaction, but since this mimics the iPod's shuffle play, it is a style of music listening I and most people are familiar with in this digital age). This is a bit more restrictive than Imeem, for instance, but last.fm has a quality catalog of good-quality mp3s uploaded directly by the label or artist (unsigned artists can upload their own songs and get paid), while Imeem relies on user-uploaded content, which has caused me problems in the past. Also, last.fm seems to have a better relationship with indie labels.
Last.fm's new service is up now, and my very brief exploration of the site was satisfactory. Things look pretty much the same, but much more music is marked as available for streaming than was the last time I visited. Out of my top ten artists, seven are available for full-length streaming (the other three are only available as thirty-second samples, which I believe means I can still hear the full tracks as part of internet radio playlists). Seven of my top eleven tracks (there's a tie for tenth place) are fully playable, and one more is available as a sample, meaning three tracks aren't available in any form. The first song I listened to, however, revealed that things aren't quite as nice as they first appeared. Robbie Fulks' "She Took a Lot of Pills (and Died)" is available, but the version is from a live performance (not his recent live album, which made my top ten of 2007 list, but a random concert available on eMusic), which was disappointing. Better than nothing, but it makes me wonder if a quick glance at the percentage of songs available may overstate how many top-quality albums are available. This problem is exacerbated by the site's focus on singles over albums; all listens to a song by the same artist are lumped together, regardless of the album. So when you look at, say, Johnny Cash's page, lots of songs are available, but you don't know what you're getting until you click. This is hardly a deal-breaker, but it would be nice to be able to choose between hearing, say, "Folsom Prison Blues" recorded at Sun Records or "Folsom Prison Blues" recorded at Folsom Prison.
I'm sure I'll be playing around with last.fm some more in the near future. There's plenty of room for improvement in their catalog, but I'm satisfied enough at first glance that I doubt I'll be going back to Imeem.