My family has been into Filk for over 15 years now. Mercedes Lackey, the writer of the Valdemar fantasy books was originally a Filker, and those books frequently have the lyrics to the filks she's written of the stories in the back. (You can find them recorded by other filkers at Firebird Arts and Music.)

Jonathan Coulton started as a Filker, but he's broken out into the big time, with a song that's the theme for G4's animated "Code Monkeys". He also did a song ("Still Alive") that's the end credits for The Orange Box's Portal game.

As someone mentioned above, Tom Smith is a famous Filker. He wasn't there because he lives in the Midwest, and doesn't come out to the West Coast often. I refer you to two Youtube versions of one of his songs, called a Boy and his Frog (about Jim Henson and Kermit). The first one has video taken from Henson's body of work, and the second is a live performance by Tom Smith.

http://www.youtube.com/watch?v=Na-xvlYMGck
http://www.youtube.com/watch?v=77f8wDf_Sdg

Oh, and have hankies ready, trust me, you'll need them.