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Updates to Left 4 Dead have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

Left 4 Dead
* *** Added Mac support for Left 4 Dead ***
* In Versus mode Tanks will spawn in stasis until a human player takes over. Tanks in stasis are invisible and invulnerable, and won't move or attack
* Tank speed is no longer reduced when on fire
* Allow players on the Infected team to manually spawn during finales
* Added a vote option toggle All-Talk
* Any Special Infected that causes a Survivor to stumble into a "ledge hang" situation will receive 50 points
* Bullets that first pass through an infected character before striking a Survivor are now ignored as friendly fire
* In Versus mode car alarms will now trigger anytime a Special Infected pulls or stumbles a Survivor into an alarm car
* Protected console commands that can be used to lag or crash servers

Source: http://steamproxy.com/steamstore/news/4564/