Commander Keen in Aliens Ate My Babysitter is the final id-developed game, released in December 1991 for MS-DOS. Originally planned to be the third episode of Goodbye, Galaxy and sixth episode overall, it was changed before development began to be a stand-alone game—stylized as Commander Keen in Aliens Ate My Babysitter!—published as a retail title through FormGen. Due to differing timelines between FormGen and Apogee, despite being numbered as the sixth episode it was developed in between the fourth and fifth; it also features a modified engine from Goodbye, Galaxy.
Source: Wikipedia