Celebrating the pending release of "The Passing" and the Left 4 Dead 1 and Left 4 Dead 2 teams colliding, we have a few other world colliding events to announce.

That's right, Internet blog readers! It's tax season here at Valve, and we need to CLEAR OUT our digital warehouses! What does that mean for you? How about the award-winning Left 4 Dead 2, now available for 50% off the original price? (March 23rd to March 25th)

While we are finishing up on the new L4D2 DLC "The Passing", we wanted to take a minute and talk about the process we use when designing maps.

But before we jump into that, we also wanted to give you a heads up to Watch Spike TV's GTTV this Friday night (March 19th) for more information on the passing, including the poster tagline.

We like the platform wars in the forums so much that we've decided to add a new platform to the fray. Later this spring, we'll be releasing both Left 4 Dead 1 and 2 on the OSX operating system for Mac computers, all through Steam.

In the first two months of Left 4 Dead 2's release, 28,981,249,043 zombies have been shot, bludgeoned, chainsawed and killed — or, for our younger readers, taken to a farm where they can frolic and shuffle around forever.

You read that correctly: 28 BILLION. To put that number in perspective:

With our last update having come out a few weeks ago and the next one a few weeks out, we wanted to let people know what we are working on. In addition to "The Passing" DLC, we are working on some other updates coming for both PC and 360. While not a complete list by any means, we wanted to give a heads up on a few Versus changes coming.

For this weekend only, Microsoft is hosting a free multiplayer weekend for Left 4 Dead 2. What does this mean? You don't need to have an Xbox Live Gold account to access Left 4 Dead 2's multiplayer features. That's right: You will be upgraded from Silver to Gold for free. You can go online alone or with a split-screen friend. This event starts at 5pm GMT Friday, January 15th and ends at 5pm on Sunday the 18th.

We are releasing the Left 4 Dead 2 Authoring Tools and Left 4 Dead 2 Add-on Support today.

PLAYERS: If you are not looking to design but play custom campaigns all you need to do is install the add-on support package. Just go into steam and open up the Tools tab. Look for the entry labeled Left 4 Dead 2 - Add-on Support click and install it. This download is sizable, but will update your L4D2 installation to allow running ANY map, including custom maps made for the original Left 4 Dead. Once installed, you do not need to run this application, it will always be active.

We will be updating the Left 4 Dead 2 game UI to include links to this package when applicable but for now please just download it directly. If you see any missing textures or models from your favorite map let us know in the forums.

DESIGNERS: Besides also downloading the above file you will need to download the Left 4 Dead 2 - Authoring Tools. Downloading it will add applications, utilities, and example files that are useful for building L4D2 game add-ons. These are similar to the tools released for Left 4 Dead, but include many updates and new features. Among the most useful are:

Gamemode Logic: Designers can now make a single map that can behave differently depending on the gamemode in which it is being played. This can dramatically reduce the file size of Campaign add-ons, as the same map can be used for Co-op, Versus, Scavenge, and Survival.

Director Logic: Maps can now alter their layout and behavior depending on how "angry" the A.I. director is. The better the Survivors are doing, the angrier the AI Director seems to get. This allows you to base variable paths and other events on the player's skill and current conditions.

Scavenge Mode Support! All the elements map authors need to support Scavenge in their maps. This includes game props as well as a full example in the form of Deadline 2, an updated tutorial campaign.

We have included an updated tutorial to help you get going and as always you can join the discussion on the map designer email list. What we want for New Years is to get to play your custom maps in Left 4 Dead 2.

For the best user experience and to take advantage of the new map and nav mesh formats, Left 4 Dead 1 Map Makers will want to compile Left 4 Dead 2 specific versions of their map in the new tool.

Happy Holidays,

The Left 4 Dead 2 Team

We have upgraded our network infrastructure to better accommodate the huge response to L4D2. 360 players will now have lag-free servers. We've also added more dedicated servers.

We will continue to monitor the situation and add resources as needed.

Thank you for your patience. Now quit reading this, get out there and kill some zombies!

Spike TV just posted their nominees for the 2009 Video Game Awards, and we're proud to announce that Left 4 Dead 2 is up for six awards—Game of the Year, Studio of the Year, Best Xbox 360 Game, Best PC Game, Best Shooter and Best Multiplayer Game. If you've been enjoying L4D2 this week, don't forget to drop into Spike's onine voting center and give us some love!