It’s a little cliché, but it’s probably Vin Diesel. He was one of the first to openly discuss roleplaying and I think that helped make roleplaying a little more mainstream. I particularly like the fact that he wrote the forward to 30 Years of Adventure, a History of Dungeons and Dragons, which is an excellent book.
Further to this though, I like the fact that Vin Diesel has actually managed to get other celebrities gaming, like Dame Judi Dench, who he ran games of D&D for when they were filming The Chronicles of Riddick. To my knowledge, to this day, Judi Dench actually runs games for her grandchildren, which is just awesome.
Beyond Vin Diesel, there are the obvious choices of Felicia Day and Wil Wheaton, but since they’ve made a career out of gaming in general, I’m not sure they’d count as their fame is now based around gaming, rather than it being something that they do as a hobby on the side. Finally there is Marilyn Manson, who reportedly carries a D20 around with him and I find it amusing that the Shock Rock Superstar sits down to play D&D in his downtime.