http://www.morecowbell.dj/
1. Choose song.
2. Set desired cowbell and walken levels.
3. ???
4. Profit
A couple ones I thought came out well, for some reason techno seems to work really well, probably the well defined beat:
http://www.morecowbell.dj/listen?id=cptuO Alice - Pogo
http://www.morecowbell.dj/listen?id=C4CWFm Toto - Africa
http://www.morecowbell.dj/listen?id=CxCEgy Daft Punk - (Explore the Space) Around the World
http://www.morecowbell.dj/listen?id=C3uqLc The Picard Song
Edit: Someone uploaded Don't Fear the Reaper 3 times, each time adding 100% cowbell.
http://www.morecowbell.dj/listen?id=DzC32O