Omnispeak (game engine)
Omnispeak is a game engine created by sulix, NY00123 and lemm. It is billed as 'an open-source reimplementation of Commander Keen episodes 4, 5, and 6' and aims to be a bug-for-bug compatible version of Keens 4–6 for modern systems. It is based on several reverse-engineering efforts, and supports features such as the cyan overscan border which are not supported by DOSBox.
It's possible to play through all of Keen 4, 5, and 6 under Omnispeak, and saved games from the original DOS games can be loaded by Omnispeak and vice-versa. The data files from a supported version of Keen are required to play: Omnispeak supports version 1.4 of Keens 4–6, and version 1.5 of Keen 6.
While Omnispeak does not officially support playing mods, it is possible to play simple mods by replacing some file and/or modifying the source code: PCKF Discussion.