In an evil region or country, what is the motivation for people to follow evil gods? I get the whole worshiping-out-of-fear thing and the idea of appeasement, but are there any other models of society where people follow evil gods willingly?
Some evil gods, such as Nerull, only seem to lend themselves to secret cults. If you look through the list of gods in 3.5 PHB none of them seem to be the sort of gods any societies would openly worship. Hextor is all very well as a god if you are warrior, but what does everyone else do?
The same can be said of Vecna and Erythnul. Who is going to worship Gruumsh apart from orcs?
The idea of secret cults seems okay, but if you want to have your world divided up into regions where good gods hold sway in one place and evil gods hold sway in another place who are the general (non-secret) temples that people pray to and why?
The World of Greyhawk campaign setting has a considerable amount of information on the default pantheon appearing in D&D 3.5 and how evil deities are followed in the world. For specific lore on the Greyhawk deities, the Living Greyhawk Gazeteer has information on these, as does the Greyhawk Wiki (for example, its article on Vecna).
These might also serve as an example for other settings.
Hextor's religion was popular in the former Great Kingdom. As a god of conquest, his religion was successful in the mainstream. His religion is described further in Dragon #356. Erythnul, god of slaughter, is popular among the humans in some wilder regions; in both cases, these deities represent the dominant cultural values of those regions at that time. Not all kingdoms are necessarily good-aligned or well-organized.
Many evil religions are specifically dangerous, so their cults are forbidden in most places. Vecna and Tharizdun are such deities. In the Free City of Greyhawk, being a follower of Vecna is a capital offence. Vecna's cult is described further in Dragon #348.
Some evil deities are the racial deities of some specific race, such as Gruumsh for orcs. Those deities are worshipped anywhere that such a race goes.
Iuz the Old (a deity not mentioned in the core rulebooks) actually resides on the material plane and commands a vast and evil kingdom. Most deities in the World of Greyhawk are forbidden from doing this, but Iuz if I remember correctly was born on that plane and is thus allowed to reside there. Evil deities actually ruling kingdoms personally is rare.
Some human peoples have one or more evil deities in their pantheon. Syrul of the Suel pantheon is followed by the Scarlet Brotherhood; they are an evil organization with their own territory, so this is a mix of the "evil cult" and "evil kingdom" tropes we've seen before.
Some less popular but less destructive evil deities are followed personally by a few individuals. Beltar, for example, goddess of caves, is followed by tomb raiders. Such religions are quieter but more tolerated than the likes of Vecna's cult.
Something you also see is major temples of evil deities tolerated in larger settlements, often for historic reasons. For example, Kalstrand has a temple of Hextor despite Zilchus being the kingdom's official religion. The temple predates the founding of the modern kingdom and hails from an era when Hextor was a dominant religion, and Overking Xavener had to make some concessions to the local major power groups to secure his own monarchy.
In general, evil deities will be openly supported if the evil that they do is out of sight of the population. This could mean that there is a "forbidden city" of debauchery, murder and the like occur but it is out of the view of the everyday citizen. It could also mean that all of the bad stuff happens to outsiders. Why would you care if the government tortures and murders people so long as it is someone else?
All those governments have to do is provide a safe and stable environment for the everyday person.
To gain power, someone must show that they "qualify" by performing evil acts and that they are smart by performing those acts on either outsiders, those who won't be missed or in a way that keeps them from getting caught.
Also, if evil acts lead to power many people will perform evil acts to gain power.
One important reason cultists worship evil gods is to get power. If you worship Pelor, maybe you'll get healing magic, provided that your heart is true. But evil gods frequently have some sort of plot to incarnate their avatar into the real world, after which their cultists will gain ultimate power.
Fear and appeasement are also valid motivations. But one final motivation is insanity. In some versions of Temple of Elemental Evil, it's narrated that contact with Tharizdun actually drives people insane, which is the only reason they continue to worship it.
For the population following this religion it wouldn't be evil. You have live all your life under this teachings and isn't evil, it's how life is.
Take by example the old Maya Empire, in this culture the religion demanded human sacrifice to appease the gods, it wasn't evil for them; it was something that has to be done.
In some other context eating the hearth of your enemy to absorb his powers is something very real that still occurs today and for the people doing it isn't evil just what needs to be done to succeed and survive.
Temples would exist for any deity without an issue. Even more in a setting like DnD where you have "visual confirmation" of their existence.