The Official Renly Baratheon in Game of Thrones is Gethin Anthony.
Update June 2011: Gethin Anthony has appeared in several tv shows before being casted as Renly. To be honest, I don't like the direction they took regarding Renly in the tv show. As I recall in the book, he was a bit of a ladies man and a real charmer. Maybe I read it wrong - perhaps the ladies swooned over him and that was all but it certainly wasn't obvious that he was gay. There may have been subtle hints that he and Loras Tyrell may have been really "close" but not to that point where they had scenes like they had in the tv show.
Renly Baratheon is the younger of Robert's brothers, the Lord of Storm's End, and Master of Laws on Robert's Small Council. He is a handsome and charismatic man, winning friends easily, but also strikes some people as frivolous.
Though Renly is Robert's youngest brother, Robert gave the Baratheon seat of Storm's End to him rather than Stannis. The lands of Storm's End are considerably larger and richer than those of Stannis's Dragonstone, giving Renly more power than his older brother.
Renly Baratheon in A Song of Ice and Fire is supposed to be a younger version of King Robert and throughout the book I imagined him to be the very handsome Michael Pare during his younger years in Eddie & the Cruisers and Streets of Fire. But for the sake of who's young and pretty boyish enough today to play this character, if HBO does decide to make a TV show out of the books, then I think Jason Ackles from Supernatural will do quite well as Renly Baratheon.