I use Greyhawk for the main fantasy world that my D20 Modern world is mystically linked to.  I explain the six human races on Oerth as the descendants of those brought over from Earth millenia ago.  There is a time dilation between the two planes-if you spend one time increment on Oerth, the next higher increment passes on Earth (i.e. if 2,000 years passes on Oerth, 20,000 years pass on Earth).  Also, in major cities (populations of 25,000+) there is a secret technomagic black market.  Those who use it are sworn to secrecy upon threat of death, and the ambient tech level is just shy of steampunk, thanks to dwarves and gnomes, but that level of technology stays mostly within their enclaves.

I run two campaigns that are offical playtests for the upcoming Bureau 13 d20 game.  The teams in my two campaigns are "Omega teams", teams of Bureau agents who are gathered to handle "Omega-level threats" (such as interplanetary and extradimensional invasions).  The Omega teams, one on Earth and one on Oerth, often act together.  The Oerth team was formed to keep any technology higher than what can be normally made or extrapolated from the general populace.  Mostly comprised of Oerthers, this Team is trying to stop an interdimensional conglomerate from taking over both Earth and Oerth.  I run many of the old GH modules, altered to fit my campaign (The Team had a major advantage in the Expedition to the Barrier Peaks adventure).  The Oerth campaign is based in Geoff, CY 591.  The aforementioned extradimensional conglomerate has found and conquered the Forgotten City in the Sea of Dust, and has aided many of the villains of the milieu, or at least, agitated events in Oerth's history.

I try to snag anything I can on GH, to make my variant campaign better, tapping into ANY source on Oerth's history I can get my hands on.  You guys have no Earthly (or Oerthly clue, for that matter LOL) how much easier my job has become.

Thank you.