2008's Left 4 Dead, by Valve, was that rare game that made you feel truly like an action hero, made you trust and rely on your friends, and utterly redefined a genre. It wasn't just that you were surviving a horrifying reality with your guns and your wits--it was that alone, you were toast. You needed your three friends to help snipe enemy smokers (zombies with constricting tongues), to cover your back when the horde charged, to heal you when you needed it. As you traversed across dark-lit settings in search of the next safe house or rescue point, you knew that around every corner randomly generated danger lurked. Requiring equal-parts strategy, cooperation, and reflexes, Left 4 Dead produced a new kind of game: the cooperative multiplayer first person survival horror. Genre aside, the game was brilliant.

Now Mac users can enjoy the original Left 4 Dead missions, the Left 4 Dead 2 campaigns, and the new downloadable missions featuring both casts. , that's more zombie killing than you can shake a machete at.

Speaking of bladed weapons, Left 4 Dead 2 now offers melee weapons. Machetes, frying pans, katanas, cricket bats--all the iconic zombie-killing weapons are at your disposal. The player can even grab a chainsaw for more bloody carnage (though due to a gas-limitation, you can't use it infinitely).

Unlike the original, Left 4 Dead 2 actually has a linear plot. The four survivors this time are trying to escape the zombie apocalypse in the Deep South. Starting around Savannah, Georgia, the four fight their way through a shopping mall, a zombie-infested carnival, a sugar mill, flooded homes and swamps and eventually to an evacuation point in New Orleans.

Fans of the original game were sad to see that Bill, Francis, Louis, and Zoe weren't in the sequel, but Valve has recently released downloadable content with new campaigns for the original cast. They're a mixed bag (The Passing had some great segments but the climax of The Sacrifice could have had some more epic set pieces) but reveal that Valve deeply cares about its fans and the characters it has created. You can also check out the beautifully drawn Web comic that explains the back story of all four characters and the tragic ending that is The Sacrifice.