Sorry for a bit of shameless self-promotion, but I feel that DiS has a lot to offer on the Cowbell front.

http://www.facebook.com/group.php?gid=39117602285

Here's the group I have established; we've got around 150 songs already, but none of them are nearly alt. enough!

So if you'd be so kind as to either join, or list any cowbell discoveries in this thread for inclusion into the definitive cowbell list. Cheers!