Valve Software on Monday announced that Portal 2, the sequel to its award-winning action game for Mac, PC and other platforms, is now available for pre-order. The game is expected to ship on April 18, 2011. Mac users can pre-order it through Valve’s Steam online game service for $44.99; it will also be included on the DVD version to be sold in stores.

[ad#Google Adsense 300×250 in story]”Special pre-order offers vary at participating outlets around the world and include a variety of dollars off and/or exclusive in-game content for those who reserve the game for purchase before it is released the week of April 18,” noted Valve. “These include $5 off the full price, exclusive in-game skins for the coop bots (Atlas and P-body), and more.”

One important deal to consider, if you have a PlayStation 3 game console – as previously announced, that version will support Steam Play, which will enable PlayStation 3 users to download the Mac and PC versions for free, as a benefit. That offer isn’t being extended to Xbox 360 users or to people who buy the PC or Mac versions alone.

The game once again pits you against GlaDOS, a “lethally inventive, power-mad A.I.,” as you make your way through Aperture Laboratories, using your wits and a handheld device that springs multidimensional portals into existence in walls, floors and ceilings.

Portal 2 features a wider variety of portal puzzles to solve, a more expansive story involving an expanded cast of characters (no longer are you alone), and both single and multiplayer cooperative gaming modes.