Guild Wars 2! Wait, you already knew that was a joke? OK, well, did you know that Cinematics Lead Horia Dociu posted up an article outlining his inspiration for the whole practical joke?
ArenaNet developers take not only their work but their fun very seriously. Dociu explains that his work at Valve and his love for games like Metal Gear Solid and Rogue Spear were part of the inspiration behind this April Fools' hit. He was able to get a bunch of developers involved in the project because they believed it would be fun for the fans. In today's ANet blog entry, he tells us that "the key was starting early, so that people could work on this fun stuff whenever they had free time. I also made sure not to involve any designers in hooking up actual game play, since that would have put a major dent in their schedules."
If you'd like to find out more about the good times at ArenaNet, check out the rest of the dev blog and the Commando class page.