Cyrodiil - Imperials: The way the Imperials have conquered Tamriel resembles what the Romans did some thousand years ago. They took pretty much all of Europe and some of northern Africa too. It is pretty clear to me that the Romans was an inspiration for the Imperials.
Skyrim - Nords: It's pretty clear that the Nords were based on the Scandinavians, especially the Vikings. The Nords are ruthless warriors, living in a cold, harsh environment, rich in mountains. So is northern Scandinavia (Norway and Sweden).
High Rock - Bretons: The name makes me think of the Celtic language Brezhoneg (Breton) spoken in Brittany, France. Brittany is located in north-west France; similarly, High Rock is located in north-west Tamriel. It is also possible that the name was chosen as a hint to something else Celtic. Say Scotland, Ireland, Wales or Cornwall. Maybe something to do with the British isles all together.
Hammerfell - Redguards: For some reason the Redguards make me think of Spanish people... They are most likely based on African people, implied by their dark colour. Also because they have nothing to do with the other human races, nor Atmora (the ancestral human homeland), and because they show a great capacity for surviving in hot and dry climates, much like Africa.
Elsweyr - Khajiit: In the Khajiit tongue Ta'agra, "Khaj" means "Desert" (the noun), and "iit" is used to state where one lives. Therefore, "Khajiit" can be translated into "One who lives in the desert" or possibly "Desert-dweller". This makes me think about the ancient Egyptians. They worshipped the cat, which seems to be the idea behind the Khajiit.
Black Marsh - Argonians: It is hard to determine what the Argonians were based on. It is entirely possible that it was just a wacky idea in the head of a developer.
Morrowind - Dunmer: The mer altogether are hard to place. As elves they are a part of the fantasy genre in general, and not being human decreases the chances of them being influenced by real life humans. However, the location of Morrowind relative to Tamriel (the Eastern Empire) and the environment hints that it was influenced by India, China, or something similar.
Valenwood - Bosmer: The Bosmer remind be of the tale of Robin Hood, living in the Sherwood Forest, located in England. The Bosmer seem to be the closest to the archetypical fantasy elf, living in harmony with nature and being incredibly skilled with bows.
Summerset Isle - Altmer: Altmer speech and writing was the base for the Tamrielic tongue. This makes me think of the old Greeks. The science, mathematics and language of the Greeks influenced and laid the ground for much of what we have today. This closely resembles the Altmer. The smugness of the elves, however, might not fit as well. The location of the Summerset Isle, and it being an isle, contradicts the theory about the Greeks, but what about the English? Summerset Isle resembles the British isles, and the smugness could well be an interpretation of the assumed smugness of the English.
High Rock/Hammerfell - Orcs: Well, orcs aren't based on anything. They're just orcs.