Falcon is a Fighter Simulation. If you look at the Link i mentioned above you will see a very active comunity.
The following descrpition was found on slashdot
Falcon 4.0, a landmark achievement in consumer flight simulation technology had its full source code leaked several years ago. What happened aftewards?
Nothing for several months. People went about playing Falcon 4.0 as they did before. Then a user posted a single screenshot to the combatsim.com fora. It showed the Falcon 4.0 options menu, except with some rather peculiar options-- 3dnow! support, 32 bit textures, object texture filtering, DirectX 7 support, and some others. Falcon 4.0 did not ship with support for said features, so either it was an edited screenshot or the user had modified the source code. Then the actual executable was released. It was real, the engine enhancements worked.
Development of the leaked source code exploded shortly after that. A team known as eTeam (the executable was called eFalcon) was created to work on it, devoted to closing the numerous memory leaks, and improving the overall realism and performance of the game. The improvements were incredible, bringing a game released in 1998 to a 2001 state, competitive (or far superior, which was most people's opinions) to simulations released that year. The game's publisher ignored this for a few years.
The game's publisher then put its foot down. It said that all development of the leaked source code had to be ceased. Quickly though the community reached an agreement. It managed to convince the publisher to allow continued development of the leaked source code, as long as the publisher maintained all rights to all of the community's work and was not required to compensate the actual contributors. The result was the Falcon 4.0 Unified Team [slashdot.org], composed of most of the eTeam members (not all though, some refused to join because of the constrictive agreement) as well as many from the Realism Patch group, a non-source code team focusing mostly on realism enhancements. The F4UT has succeeded in making hundreds if not thousands of changes to Falcon 4.0, ranging from technical (graphics engine, campaign engine, AI, sound engine, etc.) to gameplay (new flyable aircraft, dogfight AI improvements, numerous miscellaneous tweaks etc.) to other content (re-done textures, models, sound effects, completely new cockpit art, etc.). The F4UT finally brought Falcon 4.0 to what its original developers intended, not only simulation of F-16 combat, but a true military aviation experience taking place in a dynamic computer simulated war.
How does this relate to Half-Life 2's source code being leaked? Well, sometimes leaked source code can lead to greater things. After the Falcon 4.0 source code happenings, the full source code, including the graphics engine, network code etc. of a few simulations (Enemy Engaged Comanche Vs. Hokum, MiG Alley, maybe some others) have been released to the public. Maybe this practice could spread to other game genres.
Couldn't said it better.