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.
Simplified Spelljammer 5e Conversion
Jorphdan (the "ph" is silent) pulled together a bunch of disperate rules from a handful of 5e modules with a healthy mix of classic 2e. The video above gives a good sense of the ruleset and I've included the link to the document he mentions below. If this isn't enough for you, the rulesets just below this one are also excellent options.
Spelljammer 5e Conversion (for theater of the mind)
The supplement updates spelljammer combat rules to 5th edition. Inspired by Starfinder and Esper Genesis, the rules provide a new way to manage spelljammer combat, streamlining the hex grid system into a quicker "theater of the mind" playstyle.
Files: Spelljammer 5e "Theater of the Mind" Ship Combat Rules (2.8MB PDF)
Source: Mike Hammock of the Tales from the Glass Guarded World Podcast
Spelljammer 5e Conversions (for hex grid)
There are two options here, both trying to be faithful to AD&D 2nd Edition rulesets. The first is unfinished and abandoned but feels quite polished with images and community feedback. The second is much denser and completed. If I were to focus on grid play I would use Trickishwheat8's rules, but there are a few different feats worth looking at in the older conversion.
Spelljammer 5e Conversion - More Adventures in Space
Very much in the spirit of 5e, this is a slimmed down system focusing on the 5e vehicular combat system. The core of the Spelljammer lore is preserved, but there are also some useful homebrew elements that helps fillout lore and gameplay, a good mix of crunch and simplicity.
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.
Spelljammer Ship Sheet
A Spelljammer Ship Sheet that is compatible with the module Ghost of Saltmarsh 5e ship rules.
Scenario: Hunt for the Deathspider
Spelljammer_Geek was nice enough to provide a scenario, Hunt for the Deathspider, they wrote for their own game to introduce the PCs to Spelljamming using rulesets above and noted in the document. If the library of scenarios grows, I may break this out into a seperate section of its own.