(Note: If you're looking for my Greyhawk links, they're right here. Castles & Crusades links are here).
Below are most of the gaming companies I have had really positive experiences or dealings with.
Oone Games: These guys know how to do some excellent mapwork.
Basic Fantasy: A free RPG largely compatible and stylistically similiar to the Basic/Expert D&D sets, with some rule changes.
Brave Halfling Publishing: Publisher of gaming material compatible with older editions of D&D and Castles & Crusades.
Burning Wheel: Enough fiddly bits and things to think about to keep anyone busy for a long, long time.
Cumberland Games: The online home of S. John Ross, awesome creator of Encounter Critical and Risus.
Dark Matter Studios: The guys behind the Epic RPG.
Evil Hat: Makers of several RPGs and the free FATE system.
Expeditious Retreat Press: XRP has producted some of my favorite RPG supplements, to include their award-winning Magical Society line.
Far Future Enterprises: Your place to get all the Traveller RPG reprints and CD-ROMs you could ever want.
Fat Dragon Games: Makers of amazing card stock miniatures, scenery, and terrain.
Fight On!: An RPG magazine focused in a broad sense on old-school gaming and games. Their list of contributors is superb.
Flying Mice: One of my favorite RPG publishers. They've done Cold Space, In Harm's Way, and IHW: Dragons, among others.
Gamescience: The best dice on earth. Link is to a store that sells them (as does Noble Knight).
Gaming Paper: The sleeper hit of 2009's Gen Con!
Goblinoid Games: Makers of Labyrinth Lord, a free RPG that is very, very close to a clone of the old D&D Moldvay Edition.
Goodman Games: Makers of Dungeon Crawl Classics and Xcrawl, as well as a small Castles & Crusades line.
HinterWelt: Known for games about squirrels, Roma Imperious, and Nebuleon. They do good work.
Iron Crown Enterprises: Makers of Rolemaster, HARP, and Spacemaster.
Judges Guild: Maker of so, so many memorable old gaming products. Partnered with Necromancer Games.
Kenzer & Company: Makers of Hackmaster, Aces & Eights, and home to Knights of the Dinner Table.
Lamentations of the Flame Princess: Crazy, metal, old school products.
The Miniature Building Authority: I'll take "awesome stuff I can't afford" for $200, Alex.
Mythmere Games: Old-school publisher of Swords & Wizardry (oe D&D simulacrum game).
Paizo: Makers of the Pathfinder RPG, as well as the excellent GameMastery line.
Palladium Books: Makers of Rifts and Palladium Fantasy, among other RPGs.
Pied Piper Publishing: Greyhawk's Robilar (Rob Kuntz, original member of the Gygax Lake Geneva Campaign) produces several old-school works.
Precis Intermedia: Home of the amazingly useful Disposable Heroes line, and RPGs Coyote Trail, Iron Gauntlets, and Two-Fisted Tales.
Rogue Games: The Rogues make one of my favorite Space/Sci-Fi RPGs, Thousand Suns, and the tremendous Colonial Gothic.
Tabletop Adventures: They advertise products that are "help for the harried GM". Are they ever.
Troll Lord Games: The last company Uncle Gary worked with, publishers of Castles & Crusades and what we have of Castle Zagyg.
Zeitgeist Games: Publishers of Dave Arneson's Blackmoor.