Traffic and Ocean's 11 were the two that first popped into my head. The Royal Tennenbaums and Magnolia had great casts, too...especially Magnolia.
I'd like to add The Thin Red Line, though I'm the only person I know that can actually sit through the film without napping. With Sean Penn, J. Caviezal, Adrien Brody, George Clooney, John Cusack, Nick Nolte, and John C. Reilly (love!)...oh, and Travolta, though I could have lived without his presence.