Including Nathan Fillion feels like a cheat, since he appeared on Firefly first, but Fillion was very much a part of Buffy's world as the dark priest Caleb in the seventh and final season of the show. Fillion was strong as the minion of the First Evil - but of course not as strong as Buffy and her gang in the end. Fillion was a known name after starring alongside Ryan Reynolds in Two Guys, a Girl, and a Pizza Place (later renamed to Two Guys and a Girl) prior to working with Whedon, but he has still had a great deal of success since his Buffy and Firefly days, with a recurring role on Desperate Housewives, the role of Captain Hammer in Dr. Horrible's Sing-A-Long Blog (the best thing Whedon has ever done) - and, of course, the starring role in ABC's Castle. And there's always hope among the Browncoats for another Firefly resurrection.