Earth or Earth-like Worlds

I'd personally use medieval age Earth if D&D characters were going to go there, since it would require the least adjustment for the PCs. The only problematic issue about using the real-world Earth would be the treatment of religion (see any D&D thread about "God" for the flame-wars over alignment...) But any history book should give you oodles of period maps and historical characters, so it's pretty much a pre-built campaign setting. If you allow PC Clerics their spells, however, the NPC priest characters should get spells too for game balance; alternatively, PCs could find that their spells just don't work... Other than deciding the alignment of various NPCs, you're good to go (class should be fairly evident by real-life professions...)

No, Masque of the Red Death is a dark gothic Victorian/steampunk-era Earth. It might not be our Earth, but it's definitely Earth. And it's been officially linked (in Dragon Magazine) with the Earth of the 2nd edition "historical reference" series - The Celts, Age of Heroes, The Crusades, The Glory of Rome, Charlemagne's Paladins, Viking Campaign Sourcebook - the same world, but in different times and places.

H.G.Wells Earth: Victorian steampunk setting with all his monsters and gadgets (and sense of impending doom): Time machines, Invisibilty serums, mad scientists that turn animals into people, Eloi and Morlocs, Martians and their tripod invasion (for a nasty twist martians DON'T die from germs, their invasion of Great Britain is succsesful and they want to expand elswhere), gigant animals and children that have grown to enormous size after eating strange substance called "Food of Gods", etc, etc...

I love ol' H.G.

Combining the worlds of H.G.Wells, Jules Verne, Arthur Conan Doyle, etc. reminds me of the League of Extraordinary Gentlemen. The Victorian era with the various fictional elements such as those created by the above-mentioned authors creates for an interesting setting.