Escapecraft

Die2nite is a great in-browser zombie game that is not very complicated and very fun. Basically, you start in a town with 40 people. The town has a bank, a well, construction sites, and the gates. I'm not going to explain the entire game, because it would take a while. Every day, you start with 6 action points. They are used to travel around the desert outside, upgrade your house, and work on construction projects. Every night at midnight (5PM EST), the zombies attack. If you are outside town and you aren't camping in a good spot, you're dead. If the town defenses aren't good enough, zombies will get in and some people may get hurt or killed. To get items for the town, you must travel the world beyond and search the zones. If 3 or more zombies are in a zone, you must either kill them, or wait until someone else comes and brings the zone under control. The whole point of the game is to survive the longest. Not as one person, but as a whole town. This is just a brief summary, go here for more http://die2nitewiki.com/wiki/Main_Page

The actual game is here: http://www.die2nite.com

This is free, by the way.

Being a browser game, I doubt anyone would think it'd cost money.

I didn't realize when you said 40 people, that it's 40 players. Or that it was a 24/7 kinda deal.

Yea, it's you and 40 other players. After each nights attack, as you said, you have 24 hours to get on and do stuff. Once you're out of AP, you wait. Then they attack again, you're AP is restored, and so on.