Nowhere is safe from these damn things.

Ha ha, they're going to eat his face

Smoking in the boys room.

OH GOD NOT THE BUBBLES!

My hands are full, otherwise I'd write back.

How many times have you cursed the game-development gods for making games that would be awesome to play with a friend single-player only? I already know the answer: way too often. Well, here in PC gaming land, when somone hands us single-player lemons, we mod them into multiplayer lemonade. Behold! Six great formerly single-player games, including Half-Life 2 and Skyrim , that can now be played with fan-made multiplayer modes.One of the earliest success stories in modders making multiplayer where there was no multiplayer before would have to be Grand Theft Auto 3. It turns out that at one time Rockstar had plans for a multiplayer mode in the PC version, but scrapped them at some point in development. Modders discovered the unused code and, with a little tweaking, made it usable. On the downside, it's pretty clear why Rockstar iced that feature: the netcode wasn't particularly good. Syncing problems were rampant, and Rockstar's PC ports have never been particularly stable in the first place. Add to the fact that most players didn't do much with it beyond deathmatches with the world's crummiest targeting system, and it really wasn't worth messing with.Multi Theft Auto, on the other hand, has none of these problems. Synchronization isn't exactly flawless, but it's far removed from the unplayable mess of earlier mods. San Andreas already offers an enormous toy box, but Multi Theft Auto adds to that considerably, with a huge collection of custom maps and modes with more constantly being added by the community. It's enough fun to just romp around in the world with a few buddies, but dedicated racing games and a stealthy assassination mode to keeps things fresh. Even stranger modes are floating around out there, on the off chance you ever wanted to see CJ trapped in a maze full of Pac Man ghosts; I still can't fathom what possessed someone to make that one. If you've ever tried a multiplayer GTA mod in the past and wrote it off as a glitchy mess, now's a good time to give it another go and see how far they've come.It'd probably be enough to simply alter a few maps and puzzles and drop multiple players into HL2's campaign, but Follow Freeman goes far beyond that. Not only does it enable co-op play for the original campaign, but also for a whole slew of other popular mods and maps. Yes, even Minerva , which is an absolute must-play if you haven't tried it yet. On top of that it adds a shop and currency system that allows the purchase of several dozen unique weapons, including all the guns from Counter-Strike. The inventory system allows players to share weapons and ammo, and your collection of guns is saved on-server so they'll carry over between campaigns.Of course, all of that is completely optional, and can be turned off if you'd prefer to just play through vanilla HL2. Having friends does make everything a bit easier, naturally, but one can always kick up the difficulty a bit. Personally, I never realized how much I'd been abusing quicksaving until I was forced to leave it behind.The O.G. Doom and its followup are best known for their deathmatch play, which sadly overshadowed the hilarious potential of cooperative play. The "monster infighting" that erupted whenever an Imp accidentally hit a Cacodemon with a fireball was nothing compared to the brawls that would break out between players who were supposed to be working together. Anyone who ever got together with a bunch of friends to run through the campaigns cooperatively understands that sometimes it's just too tempting to smoosh a buddy with a moving floor.Doom 3 went a very different route from its predecessors. By ditching the relentless running and gunning versus hordes of nasties, it offered a slower-paced, notoriously dark atmosphere, and fostered a sense of dread before unleashing its monsters. Doom 3 OpenCoop undermines the loneliness and fear, but makes up for it with something much more akin to what the earlier games in the series did so well. All its maps needed to be tweaked to allow more than one player to complete them, and the mod even scales the quantity and frequency of monsters with the number of players. Items can be acquired once by each player, and a novel respawning mechanic puts all excess heath gathered into a common pool, to be drawn from when a deceased player needs to get back into the game. One wouldn't think that such a quintessentially single-player game would work as a multiplayer campaign, but OpenCoop pulls it off while maintaining a need for solid cooperation and teamskills.Meanwhile, if you're looking for something a bit more cutthroat and competitive, Last Man Standing is a strong alternative. The goal here is still cooperative, but this hoard mode mod inflicts massive swarms of monsters on the team while a scoreboard pushes individual players to kill more than their comrades.Oh, now this is just brilliant. Just Cause 2 is a fantastic toy box to screw around in, but throw in approximately 1,000 other ziplining, stunt-diving secret agents and it's an utterly chaotic blast. Everywhere you run there's someone trying to land a boat on a blimp or pulling off a synchronized skydive. It's early days yet for this project, however -- syncing is still fairly sketchy and beta tests only happen on weekends. Also, the player base skews a bit... er, 4chan-y, so you'll have to be prepared to see a lot of people flying airplanes into buildings while making 9-11 jokes and calling each other everything but white.Still, even with its warts, this is easily one of the most promising mods out there right now. The mobility and firepower in JC2 lend themselves perfectly to a cooperative game which in its best moments is strongly reminiscent of multiplayer Crackdown.These two are pretty much the Holy Grail of multiplayer mods. Countless amateur teams have tackled them with little success, leaving the Internet littered with big promises and aborted projects. Skyrim Online, however, looks like it might actually beat the odds thanks to its large team, strong community support, and an alpha that actually sort of works. At the moment all player models are naked prisoners, and there's nothing to do but chat, but it's coming along fast since its start in December of last year. (Yeah, they didn't waste any time.) Definitely one to keep an eye on.Oblivion Online, meanwhile, is a bit further along, though it's still a private beta and has serious syncing issues. It apparently works just fine until you do something silly, like interacting with another player or opening a menu screen. But hey, it's a start! And, at this rate, it's shaping up more optimistically than that actual Elder Scrolls MMO everyone has been preemptively bitching about.: Cramming more players into games built for single-player only seems like a terrible idea at first, but looking at what these mods have pulled off so far, that whole "the more the merrier" saying seems to be right on the money. What single-player game would you most like to play in co-op?