I have included all the links I could find that had anything MM6-related. If you know of a site I've missed please send me an email with the new address and I'll include it on this page. Thanks!
English-speaking MM6 Game Information Web Sites:
Mandate Mania - An excellent broad-based informational site. Great maps, quest info, etc.
Magic Grounds - Excellent maps and quest information, although the information is not always 100% correct (about 98% of it is though). Still, it's a very good resource with a broad base of topics.
#m&m6 The Official Home Page - Not sure why they call it that, because they're not associated with 3DO that I can tell. Nonetheless they seem to have a mid-range assortment of various maps and hints.
MM6 Spoiler - Supposedly a full-game walkthrough spoiler. Unfortunately, it's all on ONE page that takes forever to load. The information seems pretty thorough but not 100% accurate. If you have a bit of time to kill there is some interesting stuff here.
KKTai's Might and Magic VI Homepage - This page seems to load really slowly for me. Content seems pretty good, nice graphics (if you can wait for them).
Bob & Rob's RPG News - A general RPG site, but they have several pages of good information about MM6, and promise to have MM7 information as it becomes available. Looks pretty well done!
International MM6 Web Sites:
IGE's Might and Magic Page - Positively the best Japanese MM6 site I've seen. They've got other games covered too.
Chinese
Might and Magic 6 Page - Only Chinese page I'm aware of. Unfortunately I can't read the characters so I don't know if it's any good or not.
Message Boards:
Official MM6 Message Board - This site gets repetitive at times due to people asking the same questions over and over again, but if you're stuck there is usually a good following of folks who will answer your questions quickly.
Deathlands Game Forum - Including MM6 and a bunch of others. Not nearly as many posts as the 3DO Message Board site.