If you have spent any amount of time around the Half Life 2 modding community then you will be no doubt aware of Zombie Panic: Source. One of the few fortunate mods that has been chosen by Valve to be distributed via Steam, Zombie Panic: Source is often considered the main Half Life 2 Zombie Mod. The last update having been in October 2009 makes this very significant news. We know that the Developers have worked tirelessly behind the scenes and the effort shows with this update.
Zombie Panic: Source 2.1 brings various changes, additions and fixes to the mod. Notably there is a new character model and four new maps with another four existing maps updated. We also get a new menu speed, head-bobbing (which can be disabled) and various other small additions which are sure to keep this mod in the top spot in terms of zombie mods.
You can check out the page on ModDB here or go download it directly from Steam here. Happy Hunting!