I've talked about this one on Facebook from time to time, but I'm running a D&D/OGL 3.5 Campaign using Fantasy Flight Games' excellent Midnight setting.
For those unfamiliar with Midnight, the mythology of the setting posits a dark god, severed from the planar home of the Gods, who falls to Earth (Aryth, in the setting) but in the process, severs the world from the rest of the Gods. This means that the only divine magic on the world is that of the priests (called Legates) of Izrador.
The fallen god, Izrador, then begins to gather the forces of darkness, to enslave the peoples of Aryth so that he can accumulate the power to return to the immaterial planes. His intentions are basically to take the life and magic of Aryth, chug it down, and then toss the world over his shoulder like an empty beer can.
Through the course of millenia, he launched three such invasions. In the first two, he was resisted and thrown back to the frozen North of the world in a desperate and bloody fight by a coalition of elves, dwarves, and humans. The third time, however, in a fight marked by the betrayal of several key allies, Izrador wins. Imagine a Middle Earth where Sauron wins, but only after much of the Fellowship betrays its own races, and you get the idea.
Midnight picks up 100 years later. The human lands have been conquered, subjugated, and humanity enslaved. Minions of Izrador ruthlessly crush opposition. Merely forging a weapon is a capital offense. Magic items are destroyed when found. Arcane magic users are hunted down and executed if too dangerous, reeducated to serve Izrador if still useful. Elves and dwarves are ruthlessly exterminated where found.
Still, some resistance remains. In the West, guarded and buttressed by the enchanted forests of Erethor, the Elves still resist, slowly losing a battle of attrition against the forces of the Dark God. In the East, the dwarves have gone underground, and still resist in places against the orcs and other foul creatures.
In the human lands, restlessness has led some to resist, forming raiding parties, offering passive resistance in the cities and towns, and more active resistance where possible.
This is where the players begin. After recovering an important artifact from dwarven lands, they have linked up with the elves. In the process of the long journey, they come upon a woman who claims to be the widow of a descendant of the last King of Central Erenland. They have escorted her to safety, and in the process, befriended her. It is here that they have begun to gather followers to coalesce into a resistance group in the shadow of the great forest of Erethor.
Against the backdrop of the next great invasion of the Elven lands, they must fight to preserve the forest, and their hard won freedom.