Even before "Half-Life 2" hits the streets this summer, its developer, Valve Software, is bringing "hackers" up-to-speed on what they'll need to dive into the long-awaited first-person shooter's code and modify it.

At the same time, the developers at Vivendi Universal Games who are building "Tribes: Vengeance" for a fourth-quarter release have already licensed a team to fiddle with the game's engine and create its own version of the game.

Epic Games -- the home of renowned first-person shooter "Unreal Tournament" -- is preparing to shell out $1 million in cash and prizes to the person or team who does the best job of changing the 2003 and 2004 versions of that game.

What's going on here? When did hacking suddenly become legit and, in fact, a big part of the marketing strategy of video game publishers and developers?

The answer is that some -- but not all -- video game companies are actively encouraging a form of hacking called "modding" (for modifying), in which gamers change parts or all of a game by altering its code, sometimes with tools supplied by the game's developer.

Modding itself is nothing new. In 1996, alterations to id Software's "Doom" resulted in modified versions, or "mods," whose popularity extended the life of "Doom" years past the point when the game's excitement would have ordinarily dwindled. Indeed, because mods cannot be played unless the gamer owns a copy of the original retail version, it became apparent that mods have a very positive effect on the sale of the games on which they are based.

Valve Software's "Half-Life" is a case in point.

"Mods absolutely helped us drive huge sales to 'Half-Life,'" said Doug Lombardi, director of marketing at the Bellevue, Wash.-based firm. "In the typical scenario, even if a game is a mega-hit, within eight to 12 months on the store shelves, it's gone. But, in the case of 'Half-Life,' our revenue stream increased year after year for the first three years of the game's life. I attribute a lot of that to three mods -- 'Day Of Defeat,' 'Team Fortress,' and 'Counter-Strike.'"

In its first year, "Half-Life" sold two million units. A year later, sales ramped up to three-and-a-half million units. And its third year saw sales of 3.8 million units. The entire "Half-Life" franchise has sold over 11 million units all told and brought in hundreds of millions of dollars, all from a game that originally sold for $49.95 and still sells for $29.95 five years after it was released.

"Modders were responsible for extending the shelf-life of 'Half-Life' years beyond what it would normally have been," observed Dan Morris, editor-in-chief of PC Gamer magazine. "In fact, the popularity of one 'Half-Life' mod -- 'Counter-Strike' -- was so great, that gamers told their friends to forget 'Half-Life' and play the mod instead. But you needed a copy of 'Half-Life' to do that. That's why it's in Valve's best interest to first sell a few million copies of 'Half-Life 2' and then grow the market twice as large with the mods that will follow the original."

The "Counter-Strike" mod went on to become a stand-alone retail product and is now the official game of the annual CyberAthlete Professional League competition. Typically mods aren't sold but are distributed as free downloads.

Such goings-on haven't gone unnoticed by other publishers. Los Angeles-based Vivendi Universal Games is building its forthcoming "Tribes Vengeance" with modders in mind, creating documentation and using tools that will simplify subsequent alterations. It has even licensed an outside team that is building a mod, "Starsiege 2845," using the "Tribes Vengeance" engine (the underlying code that drives the film's graphics and gameplay).

"We recognize how important modding has become to the success of games like 'Half-Life,'" said Chris Mahnken, who produced "Tribes Vengeance." "When 'Half-Life' came out in 1999, it was a phenomenal product, but it doesn't look that great today and its gameplay isn't all that stellar compared to other games out there. But people are still buying it, and that's because of 'Counter-Strike' and other mods which do look pretty good."

He paused, went online, and reported back: "If you go onto the GameSpy site right now you'll see that, at this very moment, 81,232 people are playing 'Counter-Strike.' I find that amazing."

The "Tribes" series, too, has had its own success story.

"We know for certain that modding has helped both Tribes 1 and Tribes 2 survive as long as they have," noted Mahnken. "Tribes 2 is just over three years old now and it's still No. 20 on the GameSpy list of the Top 40 Games. Tribes 1 -- which came out in 1998 -- is No. 32. That's six years ago, and people are still playing it. I attribute that to the fact that both are heavily moddable games. Games that are not easy to mod or that consumers chose not to mod typically don't live very long."

At Raleigh, N.C.-based Epic Games, VP Mark Rein says the reasons to support modding go far beyond just promoting sales.

"Yes, modding extends the life of our games and it helps build community around our games," Rein said. "But it also helps us find terrific talent which we hire from the modding community. At one time, half the people who worked for us were chosen because of their modding work -- level designers, artists, programmers, everything."

In fact, Steve Polge, Epic's lead programmer, was hired in 1996 from IBM where he worked as a programmer. He made some code changes to id Software's "Quake," turned it into a mod called "Reaper Bot," and attracted the attention of Epic, which brought him on-board.

Polge describes Epic's current "$1,000,000 Make Something Unreal Contest" (at http://www.makesomethingunreal.com) as an effort by his company to unearth additional programming talent from the modding pool.

But despite Epic, Valve, and Vivendi's enthusiasm, some publishers have turned a cold shoulder to modding.

"Electronic Arts, for example, believes in customization and letting you put your face on the players in their sports games," said PC Gamer's Morris. "But that customization is a far cry from (modding) and giving modders the tools to change the game completely. Of course, EA is aware of the business model that supports modding but, instead, it wants to own its brands and not dilute them. It wants to make money on the release of its games as well as on the add-ons that follow.

"Which philosophy is the better one? As a gamer," Morris added, "I have to support the philosophy of Valve founder and managing director Gabe Newell, who says that his team may never have an idea half as good as some 13-year-old who will build a mod that will generate a demand for two million copies of Valve's next game. And so Valve gives him every tool to do that. But, at the end of the day, will EA make more than Valve? Probably."

Valve product manager Erik Johnson believes, however, that "as more and more publishers and developers prove that the support of modding is a model for success, the more people will want to get some of that action. That's the way it works in the games industry."

Paul "The Game Master" Hyman was the editor-in-chief of CMP Media's GamePower. He's covered the games industry for over a dozen years. His columns for The Reporter run exclusively on the Web site.