I’ve always liked this clip of a very young Jim Morrison,
Any clips out there of other celebrities, pre-fame?
I came here to post that same film after seeing your title. But how could have always liked it; wasn’t it just discovered a few years ago? No need to respond. I just saw a nit.
Well, always since I first saw it. 
ETA: What’s your definition of “always?”
Thanks for that. I, I think I love you…
I’m forever amused by this clip of a five year old Robert Downey Jr in one of his father’s films.
Fun clip, and (bonus!) featuring some of the worst dancing ever captured on film.
Tiger Woods on TV at age 2.
A very young Sidney Crosby: http://www.youtube.com/watch?v=neO44K7Awg0&feature=related
I’m not sure if it’s entirely pre-fame, since they use the name David Bowie, but…
At age 7, Ricky Skaggs performed on TV with Flatt & Scruggs.
Foggy Mountain Special (instrumental)
Ruby, a pretty grown-up song by a tiny kid.
Can’t supply a link but. . .
There used to be a youtube clip of a “To Tell the Truth” segment with an 9-ish year old Cynthia Nixon as one of the impostors.
A more modern one, but I thought this one was neat - Stephani Germanotta, playing piano and singing for NYU’s annual talent show. She came in 3rd.
Oh, and she’s better known these days as “Lady Gaga.”
Led Zep’s Jimmy Paige first TV appearance (includiing interview)
Simon Cowell’s first TV appearance promoting a record.
many people have seen this, of course, but check out the guy mugging Woody Allen on the subway in “Bananas.”
I’d have to dig it up–and it might be online somewhere, anyway–but I have a clip of 18-year-old Jenny McCarthy doing a local (Chicago) tv spot for a junk yard.
Deep Purple MK I on Playboy After Dark. (more great dancing, Ritchie teaching Hef how to play guitar, and John Lord talking about ghosts)
The lovely Elizabeth Shue
Mythbusters once shows a clip of an old toilet paper commercial featuring a young Adam Savage.
A young Frank Zappa on the Steve Allen Show: http://www.youtube.com/watch?v=Ql_3LS_B4q0