It's very important to give this kind of geek something to do. Bad things happen otherwise.

For some reason, the market for video games appears to be filled with people who don't actually want the games that they've bought. No, what they wanted was something nearly the same as their purchase, but not quite. Welcome to the world of game modding.

According to some sources, the earliest mod was a Castle Wolfenstein chop job called Castle Smurfenstein. Quite why the author of this game felt the need to replace Nazis with Smurfs is unclear, although I suppose the Second World War would have been over rather faster if all we'd had to do was assassinate Papa Smurf. Anyway, since then barely a game has been produced that hasn't been modified extensively by its purchasers. Even console games haven't escaped, thanks to gadgets like the Action Replay, allowing gamers to modify the system memory's contents.

The difference between using an Action Replay and modding a PC game is fairly simple - an Action Replay can only modify things that are currently in the game, whereas a PC mod can introduce entirely new things. That said, sometimes Action Replays can be used to discover things that the developers put into the game but subsequently didn't use, to the embarrassment of the game's distributors; Rockstar's revelation that Grand Theft Auto: San Andreas contained an unfinished sexual mini-game (the so-called "Hot Coffee" feature) is one obvious example. (Incidentally, the outcry over that issue strikes me as very odd - the whole game requires you to kill, maim and steal from various other people, but it's unacceptable to show simulated consensual sex on-screen?)

Obviously, then, PC mods are the most flexible type, and some people have seriously gone to town on changing their games to look like they wanted. Developers, for their part, have responded in a remarkably positive way. All of the fairly recent games in the Microsoft Flight Simulator series have had an open structure explicitly designed for users to make and fly their own planes; some of the Jedi Knight series, among many others, have options in the main menu for importing mods; and then, of course, we have the Half-Life games.

Half-Life and its sequels, easily some of the most popular and critically acclaimed games ever made, are also built on well-documented engines that make it very easy for other developers to make entirely new games on top of the old ones. The classic success story here is Counter-Strike, a mod for Half-Life that was so incredibly successful, Valve (the original publishers of Half-Life) ended up bundling it with Half-Life itself. Other mods have had similar successes - Garry's Mod, built on top of Half-Life 2, is such a useful tool that it's now being sold commercially. (And it's been used for some awesome bits of fan art, too, notably the webcomic Concerned.)

As you'd expect, the open-source community has got in on the action too, as people have created open-source remakes of brilliant games like Liero. (Many of you are now saying "I've never played Liero." To you, I can only say: Your youth was not complete.) Gusanos is one such remake, and of course it has also been modded extensively. Any game that, casting you as a homicidal intelligent worm trying to kill all your opponents, also provides you with a ninja rope, a lightsaber, and the ability to shoot lightning, must be worth playing.

Ahh, wonderful.

Now that we've established quite how awesome the concept of modding really is, take a moment to wonder why it's not present to the same extent in other software. Programs like Microsoft Word do accept "add-ins"; Windows Media Player can have extra visualisations and information processors; and yet, there's very little widespread use of these functions. The only program that comes to mind with an equivalent level of extensibility is Firefox, and that's open source, so it's much more to be expected. Perhaps software publishers are afraid that they'll lose custom if they don't have control over their products; however, gamers aren't avoiding new games in favour of mods. Rather, they're enjoying them even more.

It's entirely possible that the effect of not allowing full modding is to drive customers to completely different products. Music fans who want lots of functionality (such as full Ogg Vorbis support) don't try to mod Windows Media Player, they download Winamp; customers dissatisfied with Office download OpenOffice. And, of course, there's Linux - arguably, the closed nature of Windows has been a spur for developers to look elsewhere if they want the best performance from their programs. Whether that's true or not, it's definitely something to think about.

In the meantime, I'm off to fly an X-Wing through the middle of Tower Bridge.