Free roaming PC game...?

I dont know im just looking around for a free roaming fps i guess maybe with zombie or malformed stuff. I dont want to buy anything, it has to be free it doesnt have to have zombies but would be nice, maybe something like GTA san andreas but free, with zombies... :D but yeah are there any games like this? OR freeware fps co-op game to play over LAN (local area network)