Movie cameos? Hmmm, don't know if it counts as a cameo (it was more of an unbilled supporting part) would be Bill Murray in "Tootsie". (Several actors have done this when they didn't think the billing would be suitable at that point in their careers, but it was a *good* role.)
I'll have to think about this. (Someone should link this thread into tonight's Freeper Canteen!)
TS