Matt Smith is the 11th Doctor but he's been doing it for a couple of seasons now.
David Tennant is the 10th Doctor and he will be back for the 50th anniversary.
I don't know if Christopher Eccleston, the 9th Doctor, will be back for the 50th. I heard he's not really a fan of Doctor Who.
And I would assume that the remaining living Doctor's would be in an episode too.