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.
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.
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.