Urban Dead - The First Text Based Zombie MMO
The game claims to have over a million members, though it's hard to gauge how many of those are active. I've just started playing it, and as you have a limited number of actions you can do a day it's hard to really see what it's all about until some time has passed.
But it looks interesting, and I've heard good things.
I made a character named Dan Acroyd, a basic civilian, wandered around for a while, got into a brief scuffle with a zombie and failed pretty specularly at beating him with my golf club. As my 'action points' ran out, I tried to find a safe house to hide in, but got a bit lost and ended up taking shelter in a slightly barricaded, wrecked cinema.
I don't get more points till tomorrow, but my character is still where I left him- and he can get jumped by zombies. I think he'll defend himself as per his characteristics, but I'm not quite sure.
You can join (it takes all of half a minute) and start playing on their website, http://www.urbandead.com/.
Want to see more online zombie games you can play for free? Check out my list of the best.