Maps are always of interest to me.
- Old Maps Online is a project to make over 60,000 old maps available online.
Other useful sites are simply reference materials about various topics.
- Demonicpedia should be pretty easy to explain — a wiki about demons, though I see entries I would not classify as demons. I’m not complaining, though, they’re still useful.
- Omniglot is a linguistics site, may be rather useful for developing languages while world building.
Some other pages with lists of inspiration.
- ObsidianCrane at Daily Encounter provides a list of pictures and ideas for epic inspiration. The ideas and imagery alone are worth a look, and there are also links to the artists’ galleries.