In celebration of the holiday season, Valve is offering Left 4 Dead 2 free on Steam until 1 PM Eastern Time tomorrow (Thursday 26th). The deal is part of the Steam Holiday Sale, which offers massive savings on games between December 19th and January 3rd.

Unlike the usual free game Steam weekends, which allow you to play the game for free for a limited time, if you download Left 4 Dead 2 during the promotion it will add to your library, allowing you to play it free forever. It may not be the newest game, but for the grand total of $0 it sure is good value.

Left 4 Dead 2  89 is Valve's critically acclaimed co-op first person shooter released in 2009, with a focus on killing hordes of zombie-like Infected. The title is available for Xbox 360 as well as Windows, Mac OS X and Linux, however the game is only available for free today on the latter platforms.

As well as the sale, Valve has added in a new achievement to the game that anyone can get, called "Ghost of Christmas Present". To get the achievement, you have to help another player through a campaign who obtained the game for free during this promotion.

Head to the Steam Store today to grab a copy of Left 4 Dead 2 today, although watch out for heavy server loads, which at times are making it hard to access the Store.