Suggest some free open world videogames for PC? - 1

I want to play a free open world game for PC. Please don't tell me Minecraft. I'm tired of that game. It would be better if you tell me a zombie game (like Deadrising but that game is not free) but if you don't know any zombie game it's ok, you can tell me other games.

I have 2 GB of RAM if that helps.

Skyrim
Oblivion
GTA4
Just Cause 1 and 2
Fallout 3
Fallout: New Vegas