As it enters a new century, the late-night comedy show also celebrates its silver anniversary with a slew of celebrity hosts that starts with Jerry Seinfeld and ends with Jackie Chan and includes everyone from the Rock to Jamie Foxx in between. Other highlights of "SNL's" 25th season include an 18-year-old Britney Spears -- the youngest person in the show's history to both host and appear as musical guest in the same episode.