Aiden Gillen played the "Brian Kinney" character in the UK version of Queer as Folk, his characters name was Stuart Jones. Since Queer as Folk Gillen has since taken two different roles from Stuart one of HBO's The Wire as Councilmen/Mayor Tommy Carcetti; he also plays Petry Baelish in Game of Thrones.
Charlie Hunnam played Nathan the Justin type character in the British version of Queer as Folk. Watching Hunnam play the naive/innocent Nathan and then grow up to play Lloyd in Undeclared was fun, but watching him transform into Jax on Sons of Anarchy is nothing short of brilliant.
The first few people on this list are people a lot of LGBT folks may not know, but the rest of the list falls under the surprise realm because of either the roles in small LGBT films or incredibly popular within the LGBT community.
Dean Cain, Zach Braff, Timothy Olyphant, and John Mahoney (The Broken Hearts Club).
After Dean Cain was Superman in Lois and Clark; he move on to play a shallow gay man, based on watching him the reality series where he tries to pick a date, the shallow part may not be far off, but playing gay was an interesting choice for him.
Before Zach Braff was seeing images in his head of what might be in Scrubs as a doctor, or before he wrote, directed, and starred in Garden State, he played a bleach blonde obsessed with "gym bunnies."
Timothy Olyphant may be known as the killer in Scream 2, or two different tough guys one in HBO's wild west drama Deadwood or now the bad ass Raylan Givens on Justified, but before any of these he was the romantic lead in Broken Hearts Club.
John Mahoney played the cranky non cultured dad on Frasier, but took on the complete opposite as the guide to his boys in Broken Hearts Club.
Chris Meloni made all gay men melt when he played Chris Keller in the HBO prison drama Oz. Keller's character could easily be defined as queer because he slept with both me and women on the show, but it was his relationship with Tobias Beecher that made us swoon, about two men in prison ironically. He broke that mold playing the bad ass Detective Elliot Stabler, and those without HBO never knew otherwise.
The same can be said for Michael C. Hall and his first major role as the funeral director David Fisher. David was an insecure man who struggled with his sexual orientation even though he identified as a gay man. Hall has transformed into an incredibly different character in the Showtime series Dexter where plays a forensic blood spatter expert turned serial killer who enacts vengeance on those commit crimes, these two characters could not be more different and I think many of his Dexter fans would be surprised.