As we know in the very first 90s before Wolfenstein 3D and DOOM, in an era where much software houses were not very different from a group of indie guys that have their office into a friend garage and the copyright laws were not quite yet defined, Apogee and ID realized their influential EGA/CGA platform in exclusive for PC inspired by Supermario and pop culture references: The adventures of the 8 years old Billy Blaze a kid with an IQ of 314 that fight against any kind of menace towards the universe. Even if objectively the real success arrived in 1991 with the graphic restyling of Keen Dreams/Keen 4, If you say "Commander Keen" to someone that it is the first that generally comes in mind.
Official chapters developed by Apogee-ID software (GBC created in collab with ACTIVISION but not considered canonic by Tom Hall itself)
Invasion of the Vorticons saga
Commander Keen 1: Marooned On Mars

http://www.shikadi.n...arooned_on_Mars
Shareware Licensed Download: ftp://ftp.3drealms.com/share/1keen.zip
Commander Keen 2: The Earth Explodes

http://www.shikadi.n..._Earth_Explodes
Commander Keen 3: Keen Must Die!

http://www.shikadi.n...:_Keen_Must_Die!
Keen Dreams
Commander Keen 3,5: Keen Dreams

http://www.shikadi.n...iki/Keen_Dreams
Goodbye Galaxy Saga
Commander Keen 4: Secret of the Oracle

http://www.shikadi.n...t_of_the_Oracle
Commander Keen 5: The Armageddon Machine

http://www.shikadi.n...ageddon_Machine
Aliens Ate My Babysitter
Commander Keen 6: Aliens Ate My Babysitter!

http://www.shikadi.n..._My_Baby_Sitter!
Keen GBC
Commander Keen GBC:

http://www.shikadi.n...y_Color_game%29
Fan Mods
Universe Is Toast Saga by Ceilick & various Commander Keen fans
Based on the originally planned but then dropped ID Software idea for 1992 Christmas which is supposed to have been the last trilogy of entire franchise. Since the great quality, in a partial AscendedFanon trope manner, CK community consider them a natural continuation of the series and use their events as a base to time-placing the plot of most mods.
Commander Keen 7: The Keys of Krodacia (Requires KEEN 4)

http://www.shikadi.n...eys_of_Krodacia
Download: http://www.shikadi.n....2009-03-14.zip
Commander Keen 8: Dead in the Desert (Requires KEEN 6)

http://www.shikadi.n...d_in_the_Desert
Download: http://www.shikadi.n....2011-11-23.zip
Commander Keen 9: Battle of the Brains (Requires KEEN 5)

http://www.shikadi.n...e_of_the_Brains
Download: http://www.shikadi.n....2011-11-23.zip
Useful Links
Public Commander Keen Forum
http://pckf.rigeltechnical.com/
The Commander Keen Forum managed by CK fans as well as the best modders.
Commander Keen.Com
http://www.commander-keen.com/
"This website is made and maintained by a bunch of Commander Keen fans, who are trying to provide the best information and media for all the Commander Keen episodes. You can start navigating on our site by clicking on one of the links found from the left."
Dopefish.Com
http://www.dopefish.com/
EVERYTHING you want to know about one of the most iconic CK enemy.
Commander Genius
http://clonekeenplus...e.net/download/
"Commander Genius is a software piece that interprets the Commander Keen Invasion of the Vorticons and Galaxy series. As fans and developers we try to implement new features, improve the gameplay and give players an experience that feels like playing the original game but a bit more refreshing.
Not just Windows, Linux, and MacOS but other platforms are supported as well!"
It even works with some mods (consult the table http://www.shikadi.n...ommander_Genius) and it is provided of an High Quality Pack https://sourceforge....26.zip/download that improves visual and acustic experience.