included XAsteroids, XBattle, XBoing, X-Bomber, XConq, XDigger, XEmeraldia, XGalaga, XGammon, XLander, XLife, XMahjong, XMine, XSoldier, XPilot, XRobots, XRubiks, XShogi, XScavenger, XTris, XTron, and XTic. XBill is notable as one of the earliest free gaming titles to feature an activist theme of halting proprietary software adoption, later echoed in titles such as Virus Killer, Defendguin and FreedroidRPG. XEvil followed the development cycle of many early pieces of free software, having originally been developed as a university project on the Project Athena network, although it was freeware for a while. The game was also one of the first free titles to feature controversial subject matter such as graphic violence and drug use. XTux was also an early deathmatch game for Linux, featuring various free software mascots, a theme that would continue to be revisited. Rocks'n'Diamonds is another earlier free software game, and one of the first for Linux. The Freeciv project was started in 1995 and gave rise to another new style of free game development. Similar to the cooperative nature of the Linux kernel development, Freeciv was extended by many volunteers, rather than only one or two authors. It had started out as a small university student project but then branched out into its current form and is still being developed today. Freeciv also proved to be one of the earliest very popular free software games, and was among the first to be included with Linux distributions, a system commonly known now as a source of peer review or selection of quality for free gaming projects. Magazines, news sources and websites have also started noting free games, often in listings. Freeciv and other archetypes have led to the development of many other clones of popular proprietary games. Lincity was also started in 1995, despite there having been a Unix version of its namesake officially released by DUX Software in 1990. Beyond directly tying to the operating system, various free game development frameworks emerged starting with Allegro in 1990, SDL in 1998, ClanLib in 1999, OpenAL in 2000, SFML in 2007, as well as SDL 2 and Raylib in 2013. The GNU Image Manipulation Program, MyPaint, Kirta, Inkscape, Synfig, Pencil2D, Audacity, Rosegarden, OpenShot, Kdenlive, Pitivi, Blender, MakeHuman, MM3D, and other applications have provided an entire open source toolchain for creative projects. 3D games and source releases Proprietary games such as Doom and Descent brought in the age of three-dimensional games in the