Urban Dead

Urban Dead is a persistant browser based MMO which places you in the shoes of a survivor (or zombie) who is in the middle of a zombie outbreak. You must find safe places to hide and help out other survivors while being able to defend yourself and others from the zombie horde. There are 3 branches of class: scientist, military and civillian. Each of these branches contain various classes ranging from a simple lab assistant to a brave soldier. If you die, you join the undead in the hunt for flesh or alternatively, you can be revived by a scientist). You can also choose to spawn as a zombie from the beginning, though in my opinion being a zombie isn't exactly all that interesting. One of the main features of this game is the use of AP (Action Points). Each day you receive a certain amount of AP which are required for carrying at certain activities (moving around, attacking). The use of AP means that people who do not have much time on their hands will not be at a disadvantage to those who are hardcore players of the game and have lots of time to spend. This is why the game has become very popular with casual gamers (those without much time on their hands). It is important to note that the game is not very newbie friendly and new players are advised to use the Urban Dead wiki for information such as maps or character creation guides (http://wiki.urbandead.com/). There is also little to no graphical content in this game.

Pros:
100% free
Very intersting concept; not many zombie games out there
Large player base with lots of teamwork and player interaction (almost 1,000,000 registered players)
The game is not very time-consuming

Cons:
Once you use up your AP there is not much to do
Not very newbie friendly, you will probably need to read guides (use the wiki)
Not very graphical, almost entirely text-based

Play Urban Dead now! http://www.urbandead.com/