Forgot about Bill Irwin, but yes, I've also seen him in Ron Howard's adaptation of HOW THE GRINCH STOLE CHRISTMAS. Likewise, he was also in the video for Bobby McFerrin's song, "Don't Worry, Be Happy", along with Robin Williams.
I saw Michael Jeter before he was Mr. Noodle's Brother Mr. Noodle (even before he made a random appearance in a cover of, "Dance Myself to Sleep"), he was the villain in the original AIR BUD (the ONLY good movie in that series), as well as a neurotic architectural historian in MOUSEHUNT, as well as an adoption agent in this really bad Tarantino-esque movie called THURSDAY.
Kristen Chenoweth (Mr. Noodle's Sister Miss Noodle) was in that godawful movie remake of BEWITCHED.
Maya Rudolph (Mr. Noodle's Other Sister Miss Noodle), of course, was once an SNL castmember.
As someone previously brought up, it's interesting to look at Roscoe's acting career and see that even though almost everybody knows him best as Gordon, Gordon is actually pretty much playing against type, considering almost all of his other acting roles... it kind of makes me wonder if the CTW folks were ever concerned that he may be "too scary" to be on a kids show? I remember learning that CBS was initially reluctant to cast Larry Linville as Frank Burns on M*A*S*H because Larry had previously been typecasted as villains and bad guys, and thought he would be "too scary" to be on a comedy series.