Open source games are freely distributable and often cross-platform, meaning they will run on Windows, Linux and Mac OS computers. Similar to open source software, open source games must have their source code open and made freely available, licensed where the copyright holder provides the rights to study, change and distribute the software at no cost to anyone, and for any purpose.
Many open source games are included in Linux distributions by default, and often the more popular ones are also available to download and install on Windows and Mac OS as well. There are various shades of open source games available; pure open source games contain only free content and software, and is defined as "free games". Most free games are open source, however not all open source games are free software. This distinction makes sense when you consider some open source games contain proprietary non-free content.
Open source games are in general developed by small groups of people in their spare time, much like open source software. Run as volunteer projects with profit not being an objective, free game developers are typically enthusiasts and hobbyists. As a consequence, most games take many years to mature and there are very few complete high quality free games available.
A new trend to avoid the problems associated with free game development is to run open source game projects based on previously proprietary games, where the source code has been released under open source terms and licenses. As these games are already complete, the advantage lies in a continuing development model, using existing graphic and audio content open source developers can focus on bug fixing, modding (modifications and additions) and porting it to run on other platforms.
Open source companies and software developers listed here.
There are many famous and well known open source games today. Broadly speaking, we can identify four distinct categories of open source games. We will briefly outline each type and give a well known example in each.
Open engine and free content - These games are also known as pure open source games, developed under an open source license with free content that allows reuse, modification and often commercial redistribution of the game in its entirety. License types can be Public Domain, GPL, BSD and Creative Commons. A well known example is SuperTuxKart, an arcade racing game in the vein of Mario Kart, starring the Linux mascot Tux the penguin.
Open source games with own but non-free content - Games in this category are developed under an open source license, but only reuse and modification of the source code is permitted. Game content such as sound, graphics, video and other artwork is restricted and proprietary. Whole games are non-free with restrictions in re-use, depending on license. A well known example is CodeRED: Alien Arena, a FPS based on ID Software's open source game engine.
Open source remakes with non-free content from the proprietary original - Such remakes are developed under an open source license which allows only the reuse, modification and commercial distribution of the code. Game content such as artwork, data, etc. is from proprietary and non-open commercial games, therefore the derived game is also non-free. An example is CorsixTH, an open source clone of Theme Hospital, using proprietary original game data.
Proprietary developed games, later opened under varying licences - Originally developed as proprietary and commercial closed source games, at their end-of-life where no further revenue was expected, the source code was opened and released. By releasing the game under various open source licenses (free and non-free, commercial and noncommercial), the game is able to avoid becoming "abandonware", with the games community and public continuing to support it. A well known example is Homeworld, the first fully three-dimensional RTS developed by Relic Entertainment.
The brainchild of game industry veteran Julie Uhrman, the Ouya is an imminently available mass release video game console running a modified version of Android 4.1 Jelly Bean. Competing against the latest generation of well-established industry players such as the Wii U, Xbox One and PlayStation 4, the Ouya differentiates itself by being built upon open-source software technology and promotes community participation and development.
Unlike proprietary gaming systems, owners are encouraged to root (gain administrative access) their Ouya consoles without the usual penalty of voiding the warranty. Special developer editions of the console will come pre-rooted as well. In terms of hardware, the design has been intentional to ease modding (customisation and modifications), requiring only a standard screwdriver to open up the unit, thereby facilitating installation of possible hardware add-ons at a future date.
Owners of Ouya systems have the option to convert their units to development kits, ushering in an era of true community-driven gaming. Leveraging the power of user-contributed content and crowdsourced gaming, every gamer can also be a developer without the need for licensing fees. Similarly in the community spirit, all Ouya games are required to have some kind of free-to-play element, whether completely free, just starting on a free trial basis, or has in game/app purchases for upgrades, levels, or other in-game items.
Open source games for Windows: http://osswin.sourceforge.net/games.html
8 Awesome Free Open-Source Games You Can Enjoy On Windows, Mac and Linux: http://www.makeuseof.com/tag/8-awesome-free-opensource-games-enjoy-windows-mac-linux/