Looking for a "Middle" sized campaign setting.

[Aloïsius]

I think the 2e Jakandor is roughly what you need, except that it offers only two main cultures.

It's just an Island with two opposite civilizations.

 

[S'mon]

Wilderlands of High Fantasy possibly, might be a bit too big. But it's more than a city, and a lot less than a planet.

I think it's too big for what Hussar's looking for; it's much closer to eg Forgotten Realms in terms of scope.

 

[jasonbostwick]

Pathfinder APs tend to be set in roughly the size of area you're describing, I think.

This is what immediately came to mind for me. Varisia in Golarion is a geographically varied region with a bunch of independent city states. It doesn't really lend itself readily to what you are interested in though, as you can't just pick up a gazetteer - all of the information is spread across the various AP books.

 

[Klaus]

I think the upcoming Guide to the Nentir Vale will be just the right size for you.

 

[AdmundfortGeographer]

I think GAZ1 Grand Duchy of Karameikos is a good bet, it's really not much tied into the rest of Mystara and can be easily used in a different campaign world, eg replace off-stage Thyatis with off-stage Byzantium and have Karameikos be the western Balkans of your world.

Nearly any of the gazetteer books. Though GAZ1 is superior.

Going with multiple cultures angle, the Red Steel/Savage Coast works. It is a bit nearer towards Renaissance-era development for most D&D DM's taste. Intended to be cut-n-pasted into any DM's homebrew as a distant isolated region, even though it was placed upon Mystara if a DM didn't want to use it in the DM's own.

Jakandor could qualify.

 

[Hussar]

Excellent stuff everyone. Thanks a bunch.

 

[Jhaelen]

How about Dark Sun?

It's a very limited area with a bunch of insular city states that each have a very unique culture and 'feel'.

 

[Technomancer]

Bloodwood is a regional setting for Iron Heroes that sounds like what you want. Free for download at rpgnow.

 

[Voadam]

Spiros Blaak seems pretty good for this. A corner of a continent, a couple cities, a couple cultures. It was a d20 D&D setting, Green Ronin put out the print version but it was written by someone else. It includes Freeport in it as one of the pirate lands. I like it a lot.

Green Ronin's Hamunaptra also comes to mind, basically D&D fantasy Egypt though I don't think it has the multiculture thing you are looking for though it does have the Egypt thing split into three kingdoms.

Green Ronin also has their Indian psionics d20 setting as well which is sort of like a mini continent island. I forget the name of this one.

Nyambe by Atlas games is fantasy D&D Africa d20 with a ton of cultures but is Africa sized for the continent.

Neiyar is a 3.5 pdf jungle setting on an isolated island, probably very small continent sized.

 

[NewJeffCT]

Another setting you may want to consider has been around since 2E days. It's not world sized, but it is a bit bigger than a single kingdom - the Kingdoms of Kalamar from Kenzer is more like continent-sized, dominated by the former empire of Kalamar. They've had the fixed timeline since the beginning, and the current state of Kalamar itself is similar to either the Byzantine Empire or the Holy Roman Empire that arose after the fall of Rome - with Kalamar at its height similar to Rome at its height.

But, there are a bunch of different cultures, human and otherwise, though the setting does tend to be humanocentric.