One Page Spelljamming Adventure Generator

An incredible nifty ruleset for generating adventures in space that is RPG system agnostic, a few dice roles and you have have yourself the skeleton of a ready made adenture. John Huges, of the Blog of Holding, describes his process for developing this system.

D&D in Space - One Page Rules (450 KB PDF)
D&D in Space - One Page Rules (524 KB PNG)
Source: Blog of Holding

Spelljammer 5e Conversion

This 5e conversion tries to be faithful to the orginal AD&D rules as a consequence the combat system is pretty robust. It's unfinished and the creator suggests they will never probably finish it, but it's the most comprehensive and faithful ruleset to the original system for 5e I've found online.

36 Page 5e Rules Conversion (Unifinished) (32MB PDF)
GM Binder Source Code (Link to Github)
Source: Ubiquity4321

Homebrew Races

Solid 5th edition takes on some classic Spelljammer races. Kame Bit's goal is balance without sacrificing flavor. For example, the Exiled Illithid has feats they can take to gain the same skills as the version in the Monster Manual.

Giff (1.6KB PDF)
Dohwar (1.3KB PDF)
Exiled Illithid (2.6KB PDF)
Source: Kame Bit