Stars that can be serious actors when they want to be, instead of the silly stuff they normally do, and examples to back it up.

Jack Black: The Holiday
Jim Carrey: Eternal Sunshine of the Spotless Mind
Will Ferrell: Stranger Than Fiction
Ashton Kutcher: A Lot Like Love, The Butterfly Effect (thanks, Nic)
Keanu Reeves: Something's Gotta Give
Adam Sandler: Punch Drunk Love, Spanglish