Arma 2 is basically a very good combat simulator style game. It's the spiritual successor to operation flashpoint. The gaming environments are massive and the firefights are very realistic, you're often engaging enemies hundreds of meters away.
Wasteland is a mod that borrows the environment and weapons from arma but turns everything on its head. It's currently only on one server hosted by TAW.
Imagine a mod in which you're dropped into a 80 square kilometer map and you have to fight to survive. You need to scavenge for food, water, fuel, money, weapons and vehicles and pretty much everyone you encounter is hostile and trying to do the same. All the various resources and vehicles spawn randomly near urban centers and villages. Vehicles are all in various conditions and fuel levels. Weapons can be found on occasion in vehicles but mostly are purchased at any of the 6 designated gun stores located in safe zones that about a 200 meter radius.
In addition to resources, there are many defensive structures like sandbag walls and bunkers that also spawn randomly all over the map. These can be picked up by players and placed anywhere on the map to create firebases or reinforce existing structures and compounds. These structures can be transported in vehicles and all vehicles have a transport capacity that ranges from about 3 ro 100. The bigger the structure the more space it takes up. Large vehicles like supply trucks can tow other vehicles to allow for more cargo capacity or simply to return a damaged vehicle to a base so the vehicle can be repaired.
Every so often the game generates random events and ai enemies that will attack anyone. Some events include helicopter crashes, fuel trucks, enemy bases and so on. There are 3 primary factions, blufor, opfor and civilians. Generally bluefor and opfor work together in a loosely organized to very organized way. Civilians often comprise of smaller factions or sometimes work entirely alone killing anyone they encounter. Realistically nothing forces you to work with others at all but the game offers some very good incentives to work together. The only real rule is no killing in the safe zones, anyone who does that gets kicked. The serves generally runs for 12-20 hours before a refresh occurs and everything starts new. By that time, resources are almost all gone in terms of food and water.
Here's some pix from the last couple of nights of playing. Worth noting that the firebases you see are 100% player made. They all had to work as team to collect all the various pieces and construct them together to make some truly impressive firebases and compounds.