This is more fun than the original skit. There’s a good summary on Metafilter:
“Upload a song, experience the fever. Adds a synchronized cowbell and Walken track to any song. Part of The Echo Nest’s launch of its developer site where anyone can access our audio analysis, search capabilites or data feeds (tags / reviews / posts for any artist, all automatically.) The Python code of the cowbell renderer is also now available as an example in the open-source Echo Nest Remix API.”
I tried a few tracks. Some tracks obviously work better than others: Cecilia, Do What You Wanna Do, Jingo, Jitterbug (from the beginning of “Mulholland Dr.”), Killing In The Name, Midlife Crisis, Regular John, Stinkfist.