http://www.battleblocktheater.com | http://store.steampowered.com/app/238460/
Buckle Your Pants! BattleBlock Theater is now on Steam and new things have come with it.

Weapon switching!  Players can now switch between a primary and secondary weapon in game! New Cat Guard enemies throughout Story Mode. Steam Workshop integration for sharing user created levels. Steam Inventory integration for trading! (This allows users to trade and sell weapons, prisoners, gems and yarn via Steam.) X input, Direct Input, and Keyboard/Mouse support.

Check out the release date announcement here:
http://steamcommunity.com/games/238460/announcements/detail/1711676560410065494