My current campaign is set in the Grand Duchy of Karameikos. I've modified the rest of The Known World to suit my tastes: Darokin is a collection of petty, bickering city states, the Principalities of Glantri are ruled by a cabal of liches, Thyatis has been pushed to its own continent, and most of the other realms have been turned from organized kingdoms into loose collections of allied realms.
Prior to that, I ran a Scarred Lands game set in Mithril that was supposed to take a detour to Blood Bayou, but work and other commitments got in the way.
Before SL, I ran a Greyhawk game set in the March of Bissel. I used Return to the Keep on the Borderlands as the basis for a mini-campaign.
Other than D&D, I've run a Mutants & Masterminds mini-campaign, played Exalted and Nobilis, and have run Warhammer FRP, Omega World, and LotR.
The last two games I've played have been set in Greyhawk (regions in and around the Pomarj) and the Forgotten Realms (Waterdeep, with excursions to Undermountain).
I'd like to run an OA game set in Rokugan, but my Mutants & Masterminds campaign is due up for my post-summer second campaign. Maybe after the current Known World game finishes up it'll be time for some samurai action.