I don't know if these games were discussed here before so I'll start a new discussion.
There are seven official games - Commander Keen 1, 2, 3, Dreams, 4, 5, 6. They're all extremely beautiful but I can play one episodes 4, 5 and 6 because others are more difficult - in Commander Keen 1, 2 and 3 you can't save game inside of a level but have to be in the world map and in Commander Keen Dreams it's impossible to kill the enemies permanently except to the final boss. But I'd like to get over these limitations and play all seven games.
There's a dedicated community which is called "Public Commander Keen Forum". New users aren't allowed to post links so you'll have to search this in Google. They offer lots of fan-made mods and games for free.
I think it's extremely beautiful and I hope you'll note this too.
These games use only EGA graphics with resolution 320x200 and 16 colors but it doesn't make them less beautiful. Classic games are wonderful. 