The end credits music from Billy and Mandy by Gregory Hinde and Drew Neumann. Unlike the others floating around YouTube, this one isn’t earrape.
12/27/10 UPDATE: Since I got the Season 1 DVD set for Christmas, I will upload a better version of the ending music in high quality stereo since this one is in mono. Expect a download link.
GAoBaM is owned by Maxwell Atoms and Cartoon Network, not me.