Talk about deep cuts: Bono appears on a recent album called Prayer Cycle: Path To Zero, but he's not credited by name and the "song" on which he appears is not your typical piece of music.
He's credited as "Howling by B" on the title track, "Path To Zero," which runs 12 minutes long according to Amazon.com. The track mainly consists of the late Jim Morrison (yes, of The Doors) reciting his poem, "Moonshine Night" over Native American-inspired music. But, if you hop over to PublicMediaService.org, you can listen to a shorter version of the song. On that page, click the first link that says "Path to Zero 3:42 radio edit." Bono comes in at about the 2:15 mark and you'll immediately understand the "howling" credit. He howls for the final 90 seconds or so of the track.
Proceeds from the album are being donated to Global Zero, an international organization dedicated to nuclear disarmament. (Thx to Todd R. for the tip.)