Here’s the updated running total, with homebrew moving into a very narrow lead on Eberron, plus some more love for Spelljammer.

homebrew - 10
Eberron - 9
Spelljammer - 6
Planescape - 4
Mystara - 4
Forgotten Realms - 3
Dragonlance - 2
Greyhawk - 2
Nentir Vale - 2
Dark Sun - 1
Golarion - 1

This is my best guess based on the comments presented thus far, so there may be some fuzzy-wuzzy, timey-wimey variance in there.