Systems used: Fantasy Hero, D&D 3, D&D 3.5, and now my home brew Easy D20 system.
To date, most of the action takes place in:
Umerus - a landlocked Kingdom whose economic well-being is in jeopardy due to a greedy baron and encroaching orcs. It's capital city Invidia is well-known for its intrigue, gladiatorial combat, and Traitor's Gate, where the heads of traitors and smugglers are placed as a warning to those who would follow a similar profession.
The Badlands - mostly lifeless hills and a haven for smugglers. Contains a small woods, Dragon's Cave, and the Caves of Tamal, among other landmarks.
Arindel - a bustling, independent port city with evil lurking in the sewers below.
Boddynock - a mixed race region in the mountains that are home to humans, drow elves with a dark past, gnomes who like to play jokes and build strange contraptions, and formerly-enslaved dwarves who mostly keep to themselves.
Alitrem - a Kingdom of horsemen on the high plains east of the Boddynock Mountains. Most of the current PCs hail from here.
Sieresh Forest - a dominion of elves. Moonbrook is the capitol, yet still not much bigger than a village.