Everyone's favourite* Half-Life 2 zombie mod has just received a new update, just two days after the previous one. There's no mention of what 1.21b fixes, but if you plan on playing the survival horror mod online, you'll have no choice but to update.
If missed the 1.2b patch released on Feb 16, it added quite a few new features, including a special "infection" mode and a bunch of custom textures. The more curious can check out everything that changed right here. Don't worry - it's just a text file.
*For all you Zombie Master lovers out there, it's a totally cool mod as well. I'm not a hater.
Zombie Panic - A Half-Life 1 & 2 Modification [Official site]