What a find! Quite by accident, I stumbled across a Bruce Cockburn contribution to a Pete Seeger Tribute album on iTunes. I guess I'm not the only one who's found it because, at least on the UK iTunes Store, it's one of his 5 most downloaded.

If it's not on that list when you go there, search for Cockburn, click on his name, then click a box that says "See all songs by this artist" and scroll down to "Turn, Turn, Turn."

There's also a cover of "Ribbon of Darkness," from an album saluting Gordon Lightfoot.

On the USA iTMS, there's a song called "Wise Users" from an ecology benefit album, and a few collaborations.