Android, DOS, FirefoxOS, Linux, MacOS and Windows all in one..
Sensa is not an operating system, is a work environment compatible with Android © 4.4.4, Firefox OS ©, Linux Ubuntu 12.12 ©, MacOS Classic 7 ©, MS -DOS © operating systems, and Microsoft Windows 8.1 ©; whose distribution is free, and with source code available under GNU GPL 2.0 license .
Pruebas is an open source game mix of text adventure and RPG written in Microsoft QBasic (under MS-DOS 6.22) that uses keyboard. Runs natively under MS-DOS and compatible OS like Windows (use emulators like DosBox to run it under BeOS, Linux, *BSD, etc).
Scene16 is the open source 16-bit RPG World Builder based on Scenario v2.9, written by David A Wicker.