One campaign setting I've not seen mentioned here is Fantasy Flight Game's Dragonstar
When our 3.0 characters reached epic level, we jumped over to the Dragonstar universe and I enjoyed it very very
much! It's the perfect blend of sci-fi and fantasy - for instance: Starship engines powered by arcane teleportation spells.
Unfortunately, most of my players adamantly refuse to even consider any sort of sci-fi campaign. Curse them!
Otherwise, I'd have to go with the World of Aereth
as my current fave.