I created my first mashup using last.fm api. Its called FriendCompare.Here is a short description about it :

Friendcompare lets you compare your music tastes with your friends, neighbours or any other user on last.fm. It gives your compatability level,
common top or recently played tracks, artists, albums and tags. See whats similar between you and your friends as well as whats different and in the
process get to know your friends musical tastes better!

Here is the link to it --> http://www.ankitahuja.com/apps/lastfm