Smutefy
Smutefy is a small menu bar application that automatically detects and mute Spotify ads.
How works
Spotify uses growl to show a notification when a song changes. Smutefy uses growl log to detect ads and automatically changes the audio interface to mute the ad.
Current status
Smutefy only works in Mac OS X 10.5 and 10.6
More information: http://smutefy.inacho.es