Combining all other types of player content, the most complex creations are referred to as "total conversions." Although using the core game engine, these conversions replace the majority of the art and levels - and sometimes the gameplay - to create an entirely new experience. Although sometimes accomplished by multi-talented individuals, total conversions are usually the work of a small team. The first total conversion is generally considered to be Aliens TC, a DOOM mod designed around replacing the entire game with a new campaign based on the Aliens movies.
Now You're Playing with Power
Although they originally saw it as a threat, developers are beginning to support and promote player-created content like never before. There are a lot of good reasons for that. Financially, it's a sound move - by empowering the players, developers can give their games stronger communities, extending the lifespan of their product long beyond the traditional shelf life. Counter-Strike, a total conversion based on Half Life, was released in 1999. It is still being played - widely and constantly - today, which is more than can be said of most other games released six years ago. The community, as well as the abundance of new content it generates, helps draw in new players ... all of which require a copy of the original game in order to play.
There are more reasons than just the financial, though. Supporting player-created content is also training the next generation of content designers. Unlike development in the early days of gaming, today's games are developed by teams using a combination of standard programming and art tools and specialized, often homegrown, content creation tools. Most students can acquire the standard programs through university programs (or otherwise) with relative ease, and for the low price of a game box, can have access to a professionally built engine and a variety of the specialized tools that the developers have released. With access to the tools and engine, any budding designer today can learn the fundamentals of level design and game balance well before employment. Independent level design has become a staple on many entry-level game industry resumes for this very reason.
If your sights are set a bit higher than mere content design, mod and total conversion development can be used as a stepping stone towards getting a company off the ground - or proving a design, or getting a project lead position. Like any open medium, much of the content generated is not of significant quality, but when amazing work surfaces, the community takes notice. Provide that level of quality several times, and development studios take notice as well - several total conversion teams have become development studios in their own right.
Finally, player created content can help alleviate some of the more specific demands that developers receive. Each player is looking for different things in a game, and it's impossible for any title to please everybody. Whether it is a certain character class looking for a more specialized user interface or a group of gamers looking for a more realistic experience, niche targeted content can be created by players when it wouldn't be feasible, or productive, for the developers to create it. For an independent mod developer, an audience of several hundred can be an exhilarating rush - for a development studio it's a terrible failure.
Power Without the Price
Having player content is not without its negatives. The most evident pitfall is that users are now combing through the data files like never before - and finding things they were never meant to find. The best- known incident of this sort to date is the Grand Theft Auto: San Andreas "Hot Coffee" mod, which revealed borderline pornographic mini-games and fully nude models that had been included on the game disc, but not intended for player access. The resulting mod, which enabled the mini-games, spread like wildfire across the network world, and resulted in a retroactive rating change for San Andreas from Mature to Adults Only. A less extreme example is players' discovery of unused scenes and dialogue in Knights of the Old Republic II, and their subsequent attempts to add them back into the game.