Running Multi-Threaded Open-World Game

[A_Man]

Alright, so, a friend and I are currently thinking up of ways to run a good Post-Apocalyptic pbp, that has multiple threads and all. Basically, the idea would be that all players could do what they want, in their own separate threads, or could band up togheter (which would be the only real way to survive in this world), to accomplish a specific goal.

Now, currently, we are currently searching for a system to run this in. Preferably, it should be free, since it's pbp and we hope to have a decent amount of players, so we can't expect anyone to purchase a rule book to just play a pbp (also, it helps keep this from affecting our own pockets. ). And, we really want to keep it simple, mechanics wise.

Alright, so, now that we've got the criteria down, does anyone have any suggestions on what system to run this with? I've been thinking about using fate (and using the complication system to fit with the survival aspect of the game), but, I'm afraid that may not be the best option there is.

So, yeah, any suggestions would be greatly appreciated!

[GM.Casper]

Fudge or Fate should work rather well I think. Unless you want more- if you will "gritty" rules for for the survival aspect of the game. Maybe you can add some extra rules...

Oh, and obligatory GURPS plugin. GURPS is good for everything, and has a rather deadly/realistic combat rules good for a post-apocalyptic survival.