Not only can you play music directly from last.fm, but you can also scrobble your own tracks. Unfortunately, most of the software is (without surprise) windows based.

Nuh uh!

Download, compile, and install cmus and grab post.fm. Yes, it is perl, but it works anyway. Make the script executable and add your last.fm username and password. Next, start up cmus and type:

set status_display_program=/path/to/post.fm

Start playing tunes. Super simple and it works in the background. However, the cmus interface is a little wonky and takes some time to get used to. Again, it works and there is no need for yet another clunky gui!