Why just play games when you can make them? That's the idea behind creating game modifications (or "mods," for short), an increasingly popular activity among savvy computer gamers looking to build an all-new experience using their favorite games. And we're not just talking about new maps or skins here, either. Mods often go above and beyond new levels or graphics by changing the gameplay itself (referred to as "total conversions"), and they can also add new characters, weapons, and storylines into the mix. You always need the original game to create (and, of course, play) a mod, and afterwards, the finished product is frequently distributed using the Internet.

One of the earliest game mods ever, Castle Smurfenstein.

The mod craze dates as far back as the early '80s, when a clever fan made a few adjustments to the classic game Castle Wolfenstein for the Apple IIe and ended up with "Castle Smurfenstein." But the culture surrounding game mods really began to take off a decade later with the rise of 3D shooters, thanks to id Software's groundbreaking game Doom. In fact, by the time Quake rolled out in 1996, id's renowned programmer John Carmack had seen the desire in the community to modify his games, so he provided editing tools for Quake players to make the process of creating mods even easier. Some of the best-known early Quake mods were Dave "Zoid" Kirsch's ThreeWave and Robin Walker and John Cook's Team Fortress.

"In order to create a mod, you need an engine that allows for extension and modification, something pioneered by John Carmack at id," said Gabe Newell, founder and managing director of Valve Software, the developer of Half-Life. Valve's Half-Life is probably the single most modified 3D shooter to date. In fact, Counter-Strike, the result of a collaboration between Minh "Gooseman" Le and other Half-Life mod enthusiasts around the world, is the quintessential example of how a fan-based mod can turn into an international phenomenon played by millions. To support its fans, Valve has made a 75MB Half-Life Software Developers Kit (SDK) available as a free download on its Web site. The SDK contains source code, tools, and documentation to help developers learn how to use the engine. "Even if you aren't planning on writing a mod, it's pretty interesting to look through it, as it gives a good overview on how a typical game is implemented," Newell said.

And it's not unlikely that a good mod developer's work will be ignored--many of Valve's original employees are from the mod-making community, including Robin Walker, John Cook, Dave Riller, John Guthrie, Steve Bond, and Dario Casali. Le is now a Valve employee, too. "We keep really close tabs on who is doing what in the mod world, as it is a great proving ground for talented game developers," Newell said. "Essentially, the mod scene has turned into the triple-A affiliate of the professional game development community."

What started as a Half-Life mod is now one of the most popular online games around.

The following is a look at some of the most popular PC game mods around. Two mods are highlighted for every game, but it's worth mentioning that these mods aren't necessarily the "best" mods for the games in question--this isn't an awards feature. But they are some of the most popular, and they're good examples of what can be done with enough motivation, talent, and imagination.