If you're looking for a campaign setting that will literally inundate you with usable material -- i.e., the material that you can just plunk down at the table and start playing with -- then I would recommend Ptolus.
The only drawback is that Ptolus is only a city. (It's a city you could run campaigns in for the next decade without exhausting its potential, but it's still a city.) If you want wilderness stuff, I'd probably second the Wilderness of High Fantasy (although I'm biased because I supplement that with a lot of the old school supplements).
The only drawback is that Ptolus is only a city. (It's a city you could run campaigns in for the next decade without exhausting its potential, but it's still a city.) If you want wilderness stuff, I'd probably second the Wilderness of High Fantasy (although I'm biased because I supplement that with a lot of the old school supplements).