The original Commander Keen came as a shareware with 2 additional episodes available as a purchase. About a year later Commander Keen 4 was released as a shareware with vastly improved graphics and sound effects as well as a very nice soundtrack, Commander Keen 5 was available as a purchase.
The Commander Keen series of games makes many references to Hitchhiker's Guide to the Galaxy, such as a level in CKeen 5 called Brownian Motion Inducer. Another reference is the enemies of the CKeen games, the Vorticons, which is a very similar name to the Vogons.
Commander Keen 4 is also the source of the legendary Dopefish enemy, which has made cameos in dozens of other games from Quake 3 to Sin Episode 1. This enemy in particular functions very similarly to Big Bertha in Super Mario Bros 3.