The mercs mod is coop and has a coop server http://www.dies-world.com/index.php
As apocalypse_kid has mentioned, it is already possible to play Co-op without any mods or anything to download. Firstly, setup your "server" with settings as you like it under Multiplayer > Create Online Game (or something like that) and then, before you start the game, go to start up a single player game (choose maps and map sizes for the list, setup a password, etc... the settings you made under the Multiplayer>Create Game will stand, including the password protection) and it's as simple as having your friend connect to your IP. He clicks on Multiplayer and then Connect To IP, types in your IP address, and voila... you're good to go. Now, if either of you are behind a Firewall, that changes things, but just open the correct ports (don't know what they are offhand as I'm not home right now to check... perhaps someone else can do so now), and that's it. Keep in mind that the game is really memory intensive for your system. You're not running a dedicated server, you're only hosting a game. I say this because if just a handful of friends try to connect to you, your system will become seriously bogged down... and everyone connected to you will suffer some serious lag spikes. This is best if just you and a friend or two want to tool around with some target practice against bots. But, that's all there is to it, no need for mods or anything else. Also, understand that since I'm not at home in front of my gaming computer, I may have typed some of those things wrong (like, I don't know if the button to click is "Connect to IP" or "Direct Connect to IP" or whatever) but it should be enough to get you off the ground.
I didn't make it!
Good information Ender, I play the coop mod called Mercs at a lan center and many times I don't want to go online, I'd rather just play with the friends next to me. I'll have to try that and see. If I'm lucky I can get the management to use another station to host the coop play.... So how do you find the IP of a LAN computer?? Where is it listed?
BTW the Merc Mod is a blast to play. There are around 10 servers hosting it online at any one time, you'll see them if you install the mod and search for "same version" while in Mercs. That should be the only box you have checked. Try the uber unlocked anti-tank weapon: DAO-12.
For the computer running the game that you want to connect to:
Go to Start Then Run then type in "cmd" (without quotes) type in "ipconfig" (again, without quotes) in the window that pops up You'll see a list of things there, but the one you want is "IP Address" (jot it down... that's the IP you'll want to connect to... if you're both behind the same router, there's a good chance it will look like 192.168.xxx.xxx) Type in "exit" to close that window Now, use that address from the computer you're using and you should be good to go