Timberlake, 30, looked dapper in a Tom Ford tux. But it was Biel, 30, who stole the show: The actress showed off her enviable figure in a low-cut Prada gown. She also drew attention to her impressive engagement ring. Well done on that, J.T.
But Biel and her beau certainly weren't the only A-listers to hit the red carpet. Rihanna, Cameron Diaz, Beyoncé, Sarah Jessica Parker, Gwyneth Paltrow, Scarlett Johansson, Emma Stone, Carey Mulligan, Dianna Agron, Kristen Wiig and more stars hit the fête as well.