A free Portal-themed prototype level is now available in Killing Floor, Tripwire Interactive's co-op survival/horror first-person shooter. The game itself is also currently on sale at a 50% discount. Likewise, a new Portal -themed level is available in The Ball, Tripwire's first-person action/adventure game, which is also on sale for half off. This video shows off the new level in The Ball. Both games are also part of The Potato Sack, Valve's 13-game indie bundle, which is currently 75% off.