SkyOS is a mostly POSIX compliant operating system (OS). Begun in 1996, it is slowly growing in scope. Due to its POSIX compliance, and port of the GTK+ widget toolkit, many Unix/Linux programs are ported to it: AbiWord, Gaim, some games (e.g., Quake), and it includes the core of open source GNU tools, including GCC. OS licensing is proprietary.