It's been a little while since a music related online service has come along which has made me drop everything and rush to try it out and find out more. MoreCowbell.dj is that thing. The service is a spin off of US tv show Saturday Night Live, where a special show featured Will Ferrell as Gene Frenkle, the cowbell player, and Christopher Walken as the music producer, Bruce Dickenson. It's simple but sure-to-be-massive mechanic allows users to add a variable amount of cowbell (and optional Walken) to any track they upload. They can then embed the newly modified track as a widget in their blog, social network profile etc. Here's one I made earlier:

The really exciting thing about MoreCowbell.dj is not the service itself, but the set of APIs that it's been built on. The Echonest provide a range of APIs which enable developers to build their own digital music applications without the need for expensive servers loaded with clever custom music software. Developers can use the service to analyse tracks and return important information like beats per minute and time signature which will undoubtedly lead to a whole raft of new browser based music apps.