The AD&D 2E Accessory, Monster Mythology detailed, among other pantheons, the pantheon of the Sylvan creatures. It specifically references the Seelie and Unseelie Courts there.
It says that the Seelie Court moves among the Outer Planes of Arvandor, the Beastlands, and Gladsheim at the will of Titania, Queen of the Seelie Court.
The Unseelie Court, ruled over by the Queen of Air and Darkness, is, well I'll let the text speak for itself, because it is pretty juicy:
"Deep in the lower reaches of Pandemonium, this bitter and sadistic goddess sits among a gibbering, drooling parody of Titania's realm, in her Unseelie Court..."
The gods listed as being among the Seelie Court are:
Titania: CG(NG) Greater Goddess, the Faerie Queen
Oberon: NG(N) Lesser God, Lord of Beasts, Titania's husband
Caoimhin: N(CG) Demigod, God of the Killmoulis
Damh: CN Lesser God, patron God of the Satyrs, Korreds, and Atomies
Eachthighern: CG Lesser God, Lord of Unicorns and Pegasi
Emmantiensien: CG Intermediate God, God of Treants
Fionnghuala: NG Demigoddess, Goddess of Swanmays
Nathair Sgiathach: CG Intermediate God, Patron of Faerie Dragons, Psuedodragons, Pixies, Sprites, Grigs
Skerrit: N(CN) Lesser God, God of Centaurs
Squelaiche: CN(CG) Demigod, Patron of Leprechauns
Verenestra: N(CG) Lesser Goddess, Patron of Dryads, Nymphs, and Sylphs
Then there's the Unseelie Court, with its sole representative:
Queen of Air and Darkness: CE Intermediate Goddess, Patron of evil Elves and Faeries (She's the one who is responsible for the creation of the Quicklings.)
There is more detail given about the nature of the Seelie and Unseelie Courts (mostly the Seelie), and I'm not sure if this info was made up out of whole cloth for the Monster Mythology book, or if bits and pieces of it (such as gods/goddesses) had already existed and were just expanded upon.
I know there was a Dragon issue that dealt a lot with the Faerie races (and which, IIRC, had a different origin for the Quicklings). I'd have to dig it out to say exactly what else it expanded/changed/explained.
To my knowledge, there really hasn't been anything done with Seelie/Unseelie Courts in 3rd Edition as yet, aside from a couple of articles on the Wizards site from way back when 3E first came out.
Here are those Wizards' articles:
The Unseelie Nymph by Robert Wiese
The Fey Feature archive by various authors (mostly Gwendolyn Kestrel, I believe)
The Haunted Glen by Robert Wiese
Some of these articles are in 3.0 format for mechanics, but most of them are largely fluff, so should be fine.
