I DM primarily in Greyhawk right now. I use the LG Gazetteer and the D&D Gazetteer. Oddly, I never once played Greyhawk until 3rd edition. For OD&D I used the Known World (Mystara later), for 1st ed it was only homebrew and a brief foray into Dragonlance. 2nd ed was all homebrew and Al-Qadim.
For some reason I really bought into the idea that 3rd ed was somehow a return to D&D's roots. Mechanically, no. But I really wanted the "feel" of D&D to be there, so I decided to go as D&D as it gets - Greyhawk.
One of my current campaigns is pretty standard Greyhawk, Core only, using the adventure path as a framework (also adding elements of Return to the Temple of Elemental Evil). The other I run the characters are all built from Savage Species (to fully indulge my players, and hopefully get it out of their system), it takes place to the west of Flanaess among the ruins of the Suel and Baklunish empires. I try to make this campaign feel very different, so I use monsters almost exclusively from MM2 (as a challenge to myself). It turns out to be a very fun campaign.
I am contemplating Eberron as a future possibilty, or one of the many homebrews I have, um, brewing in my head.