[While commercial products are rarely open source, many different projects have explored the possibilities of opening up the code to the community -- and in this feature, Gamasutra looks at some examples and speaks to the minds behind them.]

The relationship between creator and audience in video games is more complicated than any other creative form. Developers can be artists, teachers, entertainers, scientists, or collaborators -- and often they must fill several of those roles simultaneously.

The role of the player is no less involved: actor, viewer, historian, student, lab rat, or leader -- all are roles he or she must be ready to play. Open source games capture this fullness of purpose, showing them at their most mechanical and most creative.

Giving players access to source code has been a part of gaming's history for years, from the earliest MUDs to Tim Sweeney's ZZT. As console gaming's proprietary hardware and its closely guarded development tools slowly squeezed PC play from its central place in the industry the idea of open source play declined.

Yet, everything connected to media and technology in the last ten years has taught us the future will be collaborative and the future is actually now. In recent years even console games have begun to realize there is profit in involving players in content creation, from Far Cry 2's extensive map editor to LittleBigPlanet 2's creation tools, which were so flexible that Media Molecule designed most of it's in-house levels with them.

And through it all, an active and growing number of PC games have kept the open source flame alive in a variety of ways, from reviving a flagging title to letting new of players learn from a designers work.

Nice Idea, We'll Take It

Crossfire began in 1992 as a topdown four-player action game in the spirit of Gauntlet. Originally developed by Frank Tore Johansen, a professor at the University of Oslo, the game has survived for almost 20 years. The first version of the game had just one map but now there are more than 3,000, as well as 150 different enemy types, 15 character classes, a skill system, and a proliferation of artifacts and treasures.

"I originally came across Crossfire after I bought an old Sun workstation to use as my home system, and so was looking for games for it," Mark Wedel, current release coordinator and one of many contributing Crossfire developers, told me. "The game interested me and I started doing some development work."

"When I started working on it, in 1992 or 1993, the game was an X11 application. This meant that for each player the server would make a remote X connection to the client. This limited the games to systems running X -- and one has to remember that this time frame predates the explosion of Linux and other free Unixes, so this was a much smaller audience."

Over the years the game has survived dramatic transitions in hardware and software, both seen and unforeseen. The first major change was moving to a client/server structure, which allowed the game to take advantage of internet play in a more accessible way.

"Because of the client/server approach, gameplay itself has changed some," Wedel said. "When I first started playing, the game was largely played by people on their own systems -- there may be groups of students at universities playing on a server. But now there are several well known servers, and anyone can play on them, so there are many more multiplayer aspects of the game than there was when I first started."

Over time Wedel became the maintainer of the codebase on Crossfire, a unique position that required him to oversee and maintain all of the incoming changes contributed to the codebase by other users. "Becoming a maintainer does result in one changing what one works on to some extent -- since I would be applying patches, I would be looking at much more code, and I might rewrite some particularly old or ugly sections, which I probably would not have done if I was just a contributor," Wedel said.

It's remarkable that a game as simple as Crossfire has been able to survive for so long. Even with a limited number of variables in its system, the game is strong proof of just how enduring comparatively simple designs can be when given over to a population of interested contributors.

"There are really only so many spells, weapons, and types of armor, and at some point it becomes redundant," Wedel said. "But within the content -- which is maps, quests, sounds, images -- that is basically unlimited. I think the world itself could keep getting expanded forever and if done right, it would not seem repetitive. Doing it well so that it does not seem repetitive would be the hard part. I would liken this to books - no one says that no new books should be written because every possible story has been told - the same would be true for the maps."

Likewise, the continual churn of new technology offers potential to reach new players and reimagine some of the game's fundaments. "10 years ago, when the client was developed, no one had any real idea about smartphones that exist now," Wedel continued.

"But with smartphones now widespread, a client that actually works well with smartphones could be written, taking advantage of the different input characteristics the phone has. It is impossible to say what new technology may show up in the next 10 years that would change the way the game is played."