Over the last few months I've been working on an addin to Windows Media Center which lets you listen to Last.fm based off of the artists in your music collection (you can also enter arbitrary artists names etc.).
I wanted it to work when the user wasn't running the MceFM application within Media Center, which meant creating a background application and a normal application. That way you can listen to songs and watch your photo collection (for example).
Also, the Last.fm music streams don't include the tags normally included in mp3 files, which let you see the track name, artist, album name, etc. So what I did was to dynamically add these tags to the stream, which means that Media Center will shown all that information correctly.
All in all, a lot of fun to develop. Now I just need to see if anyone cares
More at http://www.mcefm.com/