Valve ad in PC Gamer regarding new game (HL3 maybe?) - this time for real. Check it out.

5-2=3

half life 3 delayed for another 3 years

Albert, one of valve's staff member, who said there is no ARG in the comments of the latest HL2 update post is level 13. Floor 13 is surrounded with red light, just like Albert's level circle. What could it mean? HL3 confirmed for 2013.

It's just a joke people, it was on the joke page.

I dont want Half Life 3.

Since there is a post-it with a word Terrarium on it, I'm pretty sure they are developing Alien Swarm 2.

He put it there because in America most buildings don't have a 13th floor,which meant it does not exists.source
Also it was a joke apparently X.X

Floor 13 doesn't exist in USA, this might explain why HL3 is at the 13th floor, it would explain that nothing have been announced.

Ask Gabe