I know a few: Brad Pitt, Channing Tatum, Kirsten Dunst, Renée Zellweger. There are more actors & actresses who were extras prior to reaching stardom or becoming famous. Not all are obvious unless you knew what they looked like when they’re younger or looked via IMDb. I’ve spotted Bruce Willis as an extra in two movies: The First Deadly Sin starring Frank Sinatra in an uncredited role as “man entering diner” & The Verdict starring Paul Newman as an courtroom observer (uncredited) before he became famous in Die Hard.
It’s a supporting role in a minor movie, but George Clooney in Return of the Killer Tomatoes comes to mind, where he’s a the comic relief best friend character.