Matt LeBlanc had a few episodes on Married With Children, one of which was centered around him and his father and barely had Ed O'Neill because they were testing spinoff possibilities.
I remember that. Al only comes in at the very end to steal their TV or something.
One that I'm surprised I neglected to mention was Elijah Wood as one of the kids in the Cafe '80s in Back to the Future, Part II.
There's a British film called Jude
starring Christopher Eccleston of (among other things) Doctor Who
fame. That's not remarkable. What is is a scene he has with his future Doctor Who
successor, David Tennant. http://www.youtube.com/watch?v=YgtE-5sKN20
A couple years or so before he was cast on Stargate Atlantis,
Joe Flannigan had a bit role on an episode of Tru Calling
as the ex-husband of Harrison's new girlfriend.
While I suppose he's still not famous, one actor who really made an impression on me was the big, black prisoner on the ferry in The Dark Knight.
Imagine my surprise when I saw him in prison AGAIN in Austin Powers in Goldmember.
And while it's not an actor, I've gotten pretty good at spotting other appearances of the building facade that was used for the establishing shots of Angel Investigations during the 1st season of Angel.
It serves as the exterior of the police station in Addams Family Values
and Gina Gershon drives past it in Bound.