Common Knowledge

This article details what is common knowledge in the world about various things. This will vary by culture, people will generally be aware of things that have a direct impact on their lives. For example, people in the Shadow Marches will be aware of the Gatekeepers, but most people won't be. So determining what things are common knowledge in any particular region will sometimes be down to common sense.
With that said, the people of the Five Nations can be assumed to know the following things.

Religion

The Sovereign Host is deeply ingrained into daily life, even if you don't worship them. Most people know the names and basic attributes of the Sovereign Host and the Dark Six. Similarly, everyone knows the basic story that the world was once ruled by demons, but the Sovereigns fought them and the demons were bound.
Building on that, everyone is aware of the assertion that the Silver Flame is the force that actually binds the demons. Generally, people don't know where the Silver Flame came from, and many Vassals assume that it must've been created by the Sovereigns. Followers of the Flame assert that it is a celestial force that is strengthened by noble souls.
Most of the other religions are regional. The Blood of Vol is the best known of the other faiths because of the role it played in Karrnath during the war, but outside of Karrnath most people likely think of it as some sort of Karrnathi death cult. Everyone is at least aware that druids exist, but most people don't know anything about specific druidic sects other than the Wardens of the Wood for their role in the government of the Eldeen Reaches, and the Ashbound because of reports of terrorism.
The Path of Light is unknown aside from people who have direct interaction with Kalashtar.

Evil Exists

Following on from religion, everyone knows that supernatural evil exists. There are plenty of examples of fiends, undead, aberrations, and lycanthropes causing havoc in the world. It is common knowledge that ghouls roam in graveyards, that the creepy stranger in town could be a monster, and that dangerous things crawl out of Khyber. This is why the Silver Flame exists, and templars are generally afforded some respect even by people of other faiths. It is understood that evil exists and the templars are a volunteer militia dedicated to fighting it.

Khyber

Tied to the idea that evil exists, people know that bad things come out of Khyber. They don't know the specifics, but it is generally accepted that if you find a big hole in the ground, it's probably bad news. In particular, people are aware that the Dragon Below is the source of the bad things and don't otherwise differentiate between aberrations, fiends and monstrosities. It's all just bad news. This is why cults of the Dragon Below are named that way, even though a cult of Dyrrn the Corruptor doesn't really have anything in common with a cult of Sul Khatesh. As far as most people are concerned, they are cults that worship teh big evil things, and those big evil things come from Khyber, so, cult of the Dragon Below.
Everyone knows that the overlords were demons who dominated the world a very long time ago. Regardless of religious belief, people know that the overlords are real because one of them broke free from it's bonds and raised hell in Thrane a few centuries ago. Most people have heard stories of the overlords and may know their titles. The Shadow in the Flame is the most well known, due to the aforementioned raising hell in Thrane.
Most people don't differentiate between devils, demons, and other types of fiend and these terms are used interchangeably. Similarly, most people don't know what a "daelkyr" is. Gatekeepers and people from the Mror Holds have a better idea of the differences, but for most people they're all just powerful evil things bound in Khyber.

Dragons

Everyone knows that dragons exist and that they are terrifying. There are stories of dragons guarding hoards, and people in Thrane know that the country was once ravaged by the Bane of Thrane, the dragon who killed Prince Thrane of Galifar. Most people know that Argonessen is a land of dragons, but know almost nothing about it beyond that, and the fact that people who go there don't come back.
Some people might be aware that dragons occasionally attack Aerenel, and may know that the giants of Xen'drik were destroyed by the dragons.

Planes, Moons, and Manifest Zones

Everyone in Khorvaire knows the names of the planes and the moons, and the general idea that each plane represents. The planes have an impact on the world via manifest zones and coterminous/remote phases, and people have an understanding of this. Most people won't be able to tell you the effects of a manifest zone unless they actually live near one, but they would know that, for example, Shavarath is the plane of war and could make a guess about what a Shavaran manifest zone might do.

Fey

The existence of fey is common knowledge, everyone knows that dryads and sprites exist and that they can be found near Thelanis manifest zones. Beyond this, everyone knows fairy tales, and that there is some basis to this story. So, you might know fairy tales about a specific fey creature, and that they probably exist somewhere in Thelanis. Most people don't expect to ever meet one, though, and there isn't an easy way for the average person to differentiate between an archfey or some other powerful immortal. You could easily end up having a cult of the Dragon Below that believes they are bargaining with an archfey, when they are actually dealing with a fiend.
Specific knowledge of the fey is more common in regions that are close to Thelanis manifest zones. Notably, Aundair has a lot of these manifest zones, and I'd expect that people there are more knowledgeable about the fey.

Quori and Kalashtar

Similar to knowledge of the planes, everyone knows that Dal Quor exists and that you go there when you dream. The idea that there are spirits in Dal Quor that cause bad dreams is acceptable, but very few people believe that those spirits are manipulating events on Eberron. People have had bad dreams for as long as people have been having dreams, if the nightmares were going to take over the world, why haven't they already done it?
In regards to Sarlona, the Inspired and the Kalashtar, people are generally aware that Riedra has a strict culture and that it is ruled by beings who say they channel celestial powers. Very few people have ever actually met a Riedran, even fewer will have encountered one of the Inspired. Those who have interacted with kalashtar will know that the kalashtar have been oppressed and marginalised in Riedra, but will largely assume this to be about political or religious differences. It's relatively common knowledge that people in Sarlona study some form of mind-magic, but most people don't understand how psionics are different from other forms of magic.

Spies

When people talk about spies, they're probably thinking of the King's Dark Lanterns in Breland or the Royal Eyes of Aundair. Both of these are well known intelligence agencies, similar to our own knowledge of organisations like the CIA or KGB. Most people in Khorvaire have heard of the Trust, and understand that it's some form of secret police in Zilargo, but they don't know much more than that.

Dragonmarks

The Dragonmarked houses have been providing most of the major services people use for over a thousand years. Everyone knows the names of the houses and the twelve common marks. Most people are unaware of the Mark of Death. Aberrant marks are known to be dangerous, both to the bearer and everyone around them. Most people won't know much about the War of the Mark, aside from the fact that aberrants destroyed the original city of Sharn.

Aerenel

Aerenel is isolationist and has little interest in sharing it's traditions. The elves do trade with the Five Nations though, and there's been enough immigration over the years to provide a general idea of the culture there. Most people know that Aerenel is ruled by the Undying Court, and that the court is made up of undead elves. Most people don't have an understanding of the difference between deathless and other undead.
In the Five Nations, most people will never have heard the word "Tairnadal" before and will assume that any Tairnadal elf is from Valenar. It's understood that Valenar elves are deadly warriors that are always looking for a fight and worship their ancestors, but most people don't know any specifics about the patron ancestors.

Goblins and the Dhakaani

Most people in Khorvaire know that the goblins were on he continent before humanity was, and that they had an empire that fell long ago. Generally, people don't know the name of this empire or what events lead to it's fall. Dhakaani ruins are generally recognized as being goblin creations, and it's common knowledge that many of the largest cities in Khorvaire are built on goblin foundations.

Xen'drik

It's common knowledge that Xen'drik was once home to a civilisation of giants, and most people believe that the giants were destroyed in a war against the dragons. It's known that the elves came from Xen'drik, in no small part because of the elves own traditions and recollections of history.
People who aren't proficient with History are likely unaware of any of the giant cultures specifically, or even that there was more than one, and they definitely won't know anything about the giants fighting the quori. The idea that the giants destroyed the thirteenth moon is a folk tale, but it has many forms and some people take it as fact.