Spelljammer is an AD&D 2nd Edition campaign setting published by TSR in the early 1990s. The product line was cancelled in 1993, but the heart of this high fantasy campaign lives on in the World Wide Web. Since its cancellation, Spelljammer has been adapted to many different game systems, including AD&D 1st Edition, D&D 3rd Edition and GURPS, among others.
Imagine a universe where square worlds spin around gemstone suns. Where planets lie cradled in the roots of an oak tree so vast its leaves twirl around brightly burning suns. Where ships of wood sail the void between worlds, and battle each other with catapult and ballista, spell and sword. Where an asteroid may be a safe harbor, a slaver's den, or a hungry creature eager to devour any that pass by. Where daring swashbucklers and scoundrels race for fantastic treasures and literally touch the stars. Where terrifying beasts with the power to destroy whole worlds roam.
Welcome . . . to the universe of Spelljammer!
In the Spelljammer campaign, the fantastic is possible and one is limited only by the depths of their imagination. Sailing ships, enwrapped in bubbles of air, travel empty Wildspace, moved by the power of their mystic helms. Gravity is a matter of convenience, where a captain can tour the bottom of his ship, and worlds come in all shapes and sizes. Whole solar systems are surrounded by colossal spheres made of an unbreakable, crystal-like substance to protect them from an ocean of swirling light and color, the flammable Phlogiston, which divides the void between stars.
If you're new to Spelljammer, check out the Spelljammer Basics and Fast Play Rules found in the Game Mechanics section. That article provides the basic concepts and some simple mechanics to get you up and running with 2nd Edition rules.
Spelljammer fans have also created a wealth of fiction and artwork for their favorite setting. This material will help you explore the richness and depth of the setting.
If you're interested in D&D 3e, you may be interested in the recent Dungeon/ Polyhedron (#92/#151) issue. It contains a set of stand-alone Spelljammer rules. Though the rules are only loosely related to their 2nd Edition counterparts, there's enough in the article to get you into Wildspace. More info on this issue is available at the Shadow of the Spider Moon pages.
The Spelljammer line contained quite a few core products:
AD&D Adventures in Space; Legend of Spelljammer; War Captain's Companion; Astromundi Cluster
Monstrous Compendium: Spelljammer Appendix I & II
SJA1 Wildspace; SJA2 Skulls & Crossbows; SJA3 Crystal Spheres; SJA4 Under the Dark Fist; SJS1 Goblin's Return; SJQ1 Heart of the Enemy
SJR1 Lost Ships; SJR2 Realmspace; SJR3 Dungeon Master's Screen; SJR4 Practical Planetology; SJR5 Rock of Bral; SJR6 Greyspace; SJR7 Krynnspace; SJR8 Space Lairs
CGR1 Complete Spacefarer's Handbook
Beyond the Moons; Into the Void; The Maelstrom's Eye; The Radiant Dragon; The Broken Sphere; The Ultimate Helm
A complete listing of Spelljammer products is available at the SJ Bibliography page.
The one product you must have for Spelljammer is the main "Adventures in Space" boxed set. The "War Captain's Companion" boxed set is also quite useful, as it introduces new rules and a (very simple) ship construction system. It also has short stats for every ship. (The other two boxed sets, "Astromundi Cluster" and "Legend of Spelljammer" are campaign settings, and are of limited usefulness unless you want to base your campaign around them.)
"CGR1 Complete Spacefarer's Handbook" is also very useful. It has PC versions of some monster races, various groups and organizations of wildspace and additional general rules. The two SJ Monstrous Compendium Appendices (#7 and #9) can come in handy, for obvious reasons.
"SJR1 Lost Ships" is a good module to get. It's got tons more rules, more ships, and some general SJ info for DM's (like how to get campaigns off the ground). "SJA2 Skulls & Crossbows" and "SJR8 Space Lairs" are both good sources of short adventures/encounters, and can add flavor to the campaign.