I have created a Firefox extension that submits the tracks you play at several supported radio sites, including Slacker and Pandora, to Last.fm.
I liked Gabe's solution, but the first thought in my head was "do I really want to enter my password on a third-party site?", I know that others felt the same way, so I started working on a way to send the track data without any information ever reaching a third-party server.
This extension does not require any passwords at all, and everything sent by the extension goes directly to the Last.fm servers and nowhere else.
It supports caching of the track data if the AudioScrobbler submission server is down or having issues.
It works in Firefox, Songbird and Flock 2.6.
Talk back: Please direct feature requests, and report any issues you have in the group forum.
Setup: After you install it and restart Firefox, click on the Tools menu and you will see a LastFM > item, select it and three more items will appear, click on LastFM Options to configure the extension.
Firefox 4+: To re-enable the Status bar and Tools menu, right-click near the top of the Firefox window (in a blank space next to the URL bar) and check "Add-on Bar" and "Menu bar".
Current version: 188.8.131.52 (changelog) -- File updated: February 01, 2012, 10:14:55 GMT.
Current Beta Test version: 184.108.40.206 -- File updated: February 01, 2012, 10:02:57 GMT.