First thing you saw him in

"Hotel New Hampshire" (http://www.imdb.com/title/tt0087428/). He was about 9 or 10. :-)

Can't Buy Me Love - is the first thing I saw him in, and while he was one of the more memorable things about that movie for me, I was surprised to find out many years later that the little basterd was a young Seth Green

Wonder Years - is the first thing I remember him in, every time I saw him after that it was like "hey, there's that redhead kid from the Wonder Years again"

Austin Powers - is the movie where I finally figured out what his name was

Stephen King's It

IT and RAT RACE.
I don't mean to impose, but I am the Ocean.

Rat Race.

"Whoo-ah"

Can't Hardly Wait