Posted November 21, 2011
The only thing I can think of that mostly fills your criteria is Left 4 Dead, which is everything you mentioned except it's not free. It does go through periods of deep discounts though (usually down to $3.75, that's all I paid). It's also, like most Source games, very editable (there are dozens and dozens of fan made maps and weapon skins).
As for freeware, I think you're out of luck. There are hundreds of zombie games, but the vast majority are either not FPSes, not freeware, or both. Plus it's extremely rare to get a editable freeware FPS, unless it's open source. To the best of my knowledge, I don't think a freeware open source FPS zombie game exists. I'd love to be proven wrong by someone though.
As for freeware, I think you're out of luck. There are hundreds of zombie games, but the vast majority are either not FPSes, not freeware, or both. Plus it's extremely rare to get a editable freeware FPS, unless it's open source. To the best of my knowledge, I don't think a freeware open source FPS zombie game exists. I'd love to be proven wrong by someone though.
