Rytlock's Critter Rampage
For April Fools' Day, ArenaNet devised Guild Wars 2's Super Adventure Box update that went into the actual game as content folks could play. Part of the joke involved a commercial which briefly featured a kid playing an old-school platformer. Terrific stuff, really -- and now you can be that kid! The team went all out, creating an actual playable version.
It's called Rytlock's Critter Rampage, which is an absolutely fitting name. You can play it in your browser for free over here should you not be strictly opposed to kicking rabbits in the face until they explode. While this is a fairly straightforward little game, it's a cool gesture on ArenaNet's part.