Arguably the biggest addition to Portal 2 is the new two-player co-op campaign. But, did you know that Valve was also working on competitive multiplayer as well? Valve's Erik Wolpaw told 1UP that they "had something up and running -- the best way to describe it is sort of speedball meets Portal. You know, a sports analog." Essentially, the goal was to get a ball from one end to another, using portals.

But don't expect to find this mode in the game when it ships next year. Why? "It quickly became apparent that while it's fun for about two seconds to drop portals under people and things like that, it quickly just devolves into pure chaos," Wolpaw explained. "It lost a lot of the stuff that was really entertaining about Portal."

While Valve have given up on competitive Portal-ing, that hasn't stopped the fan community from creating their own games. Team Portress, for example, mixes a Portal Gun into Team Fortress 2, with some hilarious results.