Being able to filk is a talent unto itself! I think the best one I ever heard was YEARS ago at a Doctor Who convention in Philly. A group of five people dressed as various characters from Doctor Who sang a cappela a very funny song called "Traveling Through Time", which was sung to the melody of Billy Joel's "The Longest Time". I was laughing my ass off! I wish I could remember some of the lines from it...

Doctor Who conventions. Yes. I am a geek.