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.
Nicolas Cage had a small role in Fast Times at Ridgemont High, as one of Judge Reinhold’s coworkers at the burger joint. Credited as Nicolas Coppola.
Tobey Maguire had a part in Empire Records. His scene was cut but he still appears in the credits.
Jacob Wysocki (of Dropout) has a small role in Pitch Perfect.