I've just gotten into Last.fm (as you can see in my sidebar) and I'm enjoying the site. I couldn't be arsed with the full program to monitor what music I had listened to, so I downloaded the "outdated" (so their site says) plugin for Windows Media Player instead. So far I'm impressed with it - It regularly uploads my song information with no problems at all. I don't see anything outdated about it.
I'm currently using the "Run for Cover" beta widget in my sidebar. It's ace, and displays the covers to my recently listened to tracks. I hope the author doesn't mind that I messed around with the padding and added a link to my Last.fm profile under the widget.
