A long time ago TSR was running out of setting ideas for Dungeons & Dragons. In the manner of the “Leprechaun” and “Hellraiser” franchises, eventually they settled on the one remaining unexplored world: SPAAAAAACE! Spelljammer was a gonzo, kitchen sink kind of world that allowed every other setting to fit into it, from Greyhawk to the Forgotten Realms. The clever twist was that space travel was filtered through a medieval mindset, complete with crystal spheres and phlogiston. It was both corny and brilliant.
A little too corny for 3rd Edition. There were some attempts to bring Spelljammer into 3rd Edition rules in Polyhedron, but for the most part the setting was criminally neglected. Third parties took up the mantle; Aether & Flux is the closest to the original Spelljammer setting. Like Spelljammer, there is an aether and spellcasters are required to “sail the traverse.” Unlike Spelljammer there is also Flux, a form of electrical energy that can be harnessed by mechanical flux cullers.
Spelljammer was also known for its alien races, particularly the Neogi, a cross between spiders and eels. Aether & Flux has the Ravin, an eight-legged bug-like race resembling ticks. The rest of the book is fleshed out with different types of ships – remember, since fantasy space bends the laws of physics, all manner of ships are possible. New equipment, feats, spells, ship-to-ship combat, weapons, and worlds round out the book.
Aether & Flux is basically an updated Spelljammer. While it’s not an official product, it’s the next best thing for 3rd Edition.