Ashton Kutcher? He's done phenomenally well. According to Wikipedia, Kutcher left the show in its last season due to scheduling conflicts with a movie he was making. He later married Demi Moore. Now he's a venture capitalist, after making lots of money on early technology investments.
-PJ
Ashton stayed with the show and made movies simultaneously. Topher, whose character was the son of Redd, left the show when it was at its height. I’m guessing because he didn’t like being overshadowed by Ashton.