As Microsoft was finishing a new game that lets players crossbreed animals -- as in elephants with sharks -- it gave a select group of programmers an opportunity to tinker with the product.

By the time "Impossible Creatures" came out a month later, the tinkerers, called "modders," had already developed unofficial variants that go beyond the basic game -- and released them as free downloads on the Internet.

Some of the add-ons used dinosaurs instead of present-day animals. Other "mods" had new sound effects and revamped graphics.

For years, modders have blended software savvy with creativity, breathing new life into aging games. Now, their skills are being sought for new PC games.

Modding is somewhat like souping up car engines in the old days or expanding the capacity of TiVo digital video recorders more recently. But here, the end product is shared among modders.

The modders largely do this unsolicited and uncompensated, essentially supporting a $10 billion industry with free labor. For them, it's all about the love of games, not of dollars.

Microsoft and others don't have any legal quibbles with mods and even quietly encourage them.

"We don't know what folks will create and we don't control it," said Stuart Moulder, general manager of Microsoft Game Studios. "I believe we have to embrace this dynamic as necessary to allow the fullest flowering of creativity."

Some oppose tinkering

Such an attitude contrasts with the way some manufacturers view tinkering as a threat to copy-protection schemes and bottom lines. Sony Corp., for instance, has discouraged letting owners of the robot pet Aibo teach it new tricks.

Microsoft, meanwhile, is fighting to stop the spread of mod chips that can be plugged into the Xbox video game console, turning it into a Linux computer or giving it other functions. Moulder said such hardware mods tweak systems so they can be used for unintended purposes -- as opposed to software mods, which enhance play experience.

One of software modding's biggest successes came in 1999, when modders turned Half-Life's "X-Files" world of government conspiracies and alien invasions into Counter-Strike, a multiplayer game that pits soldiers against terrorists.

Counter-Strike used the same underlying engine as Half-Life, but the mod had completely new maps, weaponry, graphics and sound effects.

Half-Life's owners liked it so much, they decided a year later to release Counter-Strike commercially. They wound end up selling 1.5 million copies, generating $40 million in sales. The Internet game remains among the most popular around.

Modders didn't mind that the owners made money from their free labor.

Corey Taffe, a 23-year-old programmer from Minneapolis, says modders are motivated mostly by creativity and curiosity.

"I wouldn't go into a mod project thinking I'm going to make some money off of it, and I doubt many modders do," Taffe said. "If it turns out that it's popular and worthy of itself being sold as a stand-alone product, all the better."

The latest big collaboration is set to debut this summer and this time modders will have a more formal role. Half-Life developer Valve Software, Dallas-based developer Ritual Entertainment Inc. and a slew of modders are working on Counter-Strike: Condition Zero, which will bring a single-player mode, along with a facelift, to the original multiplayer Counter-Strike.

About a third of Valve's 80 employees are former modders, a testament to their growing influence in the games industry.

"The mod guys are sort of getting their dues," said Valve spokesman Doug Lombardi.

Many games released today come with the same software tools used by their developers, giving anyone with the skills and desire the ability to create mods.

Modders have come to expect no less.

Moulder of Microsoft said the two days of modding classes Microsoft sponsored for "Impossible Creatures" in Vancouver, British Columbia, in November helped build a stronger relationship with the often fickle gaming community.

P.J. McNealy, an analyst with GartnerG2 in San Jose, Calif., called Microsoft's modding classes good public relations but not a good way to do business in the video games industry because it assumes modders will want to make free add-ons for games.

Still, Lombardi said, mods like Counter-Strike show that anyone with the skills can make it big in the gaming industry.

"They are the guys that are going to be making the great games of tomorrow," he said. "Gaming is still in the frontier days, there's still a chance for someone to put something together in a garage."