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.
Not an actor (?) or a movie, but a young Lady Gaga was an extra in an episode of The Sopranos. She was part of the high schoolers in the bleachers in one of Anthony’s swim meet scenes.