Garnia- The core nation of mankind on the planet, it also used to be the core of the Sidhe (Elven) empire. Most of the other nations of man are related to them in some way or another. The culture there is conservatively Celtic, at the time of the "present" the technology level is roughly equivalent to 1000 CE western Europe on Earth, but with magic.
Frodia- The frequent foil to Garnian ambitions and vice versa, the two nations share a long river border. Frodia has developed somewhat independently of Garnia, eschewing a warrior elite in favor of a Wizard-Priest elite ruling caste. They speak essentially the same language though, and share much of the same material culture otherwise.
Tirnakaur- Actually divided into eastern and western factions on their large forested penninsula, this civil war torn nation is a remnant of the original Garnian ruling dynasty that was deposed by an incursion of steppe barbarians from the north that established a new ruling dynasty.
Steppe Barbarians- A catch-all term for everyone living on the northern steppes. Periodically they get their act together and invade Garnia with overwhelming force, then establish a new Garnian dynasty. Usually the new High King doesn't maintain control of the steppe tribes for very long after "trading up" to Garnian civilization. They are also the "Parent" group for all of the worlds Celtic nations.
Wodanslund- A Saxon kingdom to the south of Frodia. Their warrior aristocracy often serves as mercenaries to the Frodians, to the point where some have considered Wodanslund to be a sub-kingdom or satellite state of Frodia. They introduced the stirrup to this world!
Khazarak- the mountain kingdoms of the Dwarves. Every year they lose a little more territory to the marauding hordes of humanoids. They form the western border of Garnian territory.
Sidhe Empire- They're still out there, their power has been shattered by the coming of man and they lost the vast majority of their territory, but every now and again they make their presence felt.
Recently Canonical and up for Review-
Mistland Clans- A confederacy of Clans that control territory northeast of Garnia to the east of the Great Rift. Iron rots there within days. The people there are fiercely independent. They are a Q-Celtic speaking people.
The City-States of the Far West- Another catch-all term, they are like Greek city-states, in that they are all independent and each have their own forms of government, they are descended from the original Celtic tribesman too. Most of the cities are in mountain valleys of the far west, south of the steppe, far from Khazarak and it's humanoid problems.
The Empire of Ming Liang (AKA- The Bright Empire)- A Chinese Empire to the East of Garnia with whom they have had some trade and occasional military friction, the rough territory between them keeps them from really fighting anything out. These guys have gone through a lot of incarnations over the years, making them Chinese was the latest attempt to make them relevant and interesting, plus I figured it would let me play Oriental Adventures campaigns without ever leaving my home campaign world, always a bonus.
Tenchuko- I never fleshed these guys out, but they were Japanese and were just off map, I wanted them in game for the same reason that I made the Bright Empire Chinese, so I could play OA in Garnia.
Altan Ordu- These guys are a Turko-Mongolian horde, they seem like just an attempt to add my Steppe Warriors to Garnia, but they actually appeared in a campaign I ran in the 1990s as mercenaries that the PCs needed to hire on and work with to halt an invasion from the steppes, this invasion was actually being led by some pretty hard-core evil dudes, so it wasn't just the run of the mill change of dynasty type of invasion, the Darkness was deeply involved.
Imperium Romanum- This is the other side of the mega-continent. I didn't really plan it this way, but in the mid 1990s I ran an AD&D Roman campaign set in a fantasy Roman Empire that I designed. There was a quasi-Mediterranean area, a goblin-conquered Egyptian Empire, recently acquired by Rome, and a large, mountainous set of Dwarven kingdoms to the east. That campaign never got the chance to play out, but I realized later on that the maps for it and the maps for Garnia matched up perfectly; so I welded the two together. I had read Harry Turtledove's "Misplaced Legion" and Jerry Pournelle's "Janissaries" by then, as well as worked on Garnia for like a decade or more, so transferring a bunch of Romans to another world wasn't exactly a new concept to me, and I always liked the idea of using real world cultures as a base for my fantasy games, clearly.
Nynorsk States- They are a bunch of Viking states. I made them descended from Viking settlements that disappeared in our world. I ran a 3e campaign there based out the trading port of Kaupangrfjord, and I made a pretty extensive Neverwinter Nights module called Norseworld based out of there. The NWN module eventually started to stray from my 3e game's canon, and my 3e game was never actually set in Garnia, but since the apparatus was pretty much the same, and they could be placed off map, I figured it couldn't hurt to put them there.
Trebizondic Empire- I wanted a Byzantine remnant. I never designed anything. These guys would be easy to cut.
Xochitli- I thought it would be cool to add some Aztec/Meso-Americans and I had an open space on the map. They were never developed in the slightest either.
Kushan- I realized that I didn't have any black people in my world and didn't want to look like a racist dick. Of course, then I never developed this part of the world either, so take that for what it's worth.
Demotic- Really just a linguistic phase of Egyptian culture, I picked this to describe the language spoken by the Goblins that conquered this alt-Egypt and were in turn conquered by Alt-Rome.
Orcish Kingdom- This has been a staple of the Garnia campaign since the first days of it's existence. The Orc Kingdom was originally called the "Royal Orc Kingdom", but about the time I turned 13 I decided Royal and Kingdom in the country's name were redundant. It needs a new name, but I am cool with them sticking around. They are the real source of tension and adventure in Garnia, they are a former territory of Garnia.
Necromancer Lands- Another staple of the campaign world, the Necromancer is a world class bad guy, exiled from Frodia to a barren wilderness he continued his studies into the dark arts and attempted to conquer the world. He was defeated, but not destroyed. When will his evil rise again? The dude is practically a living evil god with cults of followers that still hold sway over his old territory of exile.
Elf Cities- I figured that the Elves, fallen as they were must have some place where they hold sway, I never developed them though, not in the slightest. These would be cities of exiles.
Places that haven't been Canonical in so long I forgot they existed-
Asros- I have no idea what this county's story is, except that it is in between Garnia and the Bright Empire apparently on an old map Darryl found.
The Halfling Lands- I knew they had to have had a homeland somewhere, turns out it was in the southeastern corner of the Bright Empire.
Stejyn- Apparently a vassal state of Wotan, AKA Wodanslund.
The Maritime Emirates- These guys are apparently vassals of Asros, on the coast, and evidence of the existence of Arabs in my world from the earliest dates. That's funny to me because the ethnic groups that pop up time after time for NPCs in my games are Arabs, Vikings, Turks and Gaels; pretty much in that order. I have to work hard to keep the majority of NPCs having P-Celtic names. Oddly, my last Garnia campaign picked up an Italian NPC named Lorenzo too.
The Pirate Isles- Not too much information here, they're islands and apparently pirates live there.
That's all I can remember. Darryl, do you have any more? I think I covered everyone anyway.