Praxa (Czech for 'practice') is a web-app that helps you compare your skill (singing, playing an instrument) against a master recording.
Record the 'master' audio file you want to compare against and then your song (with the playback feature).
The comparison feedback is provided both through audio when playing (you can hear the 'master' tune)
and in a graphical format. Hope you like it and please send in your comments.