Here are some really great--and sometimes hard to find--research sites and tools.  You never know what oddball thing you may need to find for something you're writing...

Castles on the Web--You will not believe your eyes when you see this website.  A fantasy writer's treasure trove of information, including castle terms defined, swords and weapons information, myths & legends, heraldry, and of course, castle photos.

Lady Gryphon's Mythical Realm--Mythical beasts, people, and races represented in picture, short story, poetry, with histories.  What a fascinating site!

Legends--A one-stop shop for information on many of the legends that form the basis of fantasy tales.  Find Robin Hood, King Arthur, fairy tales and Shakespeare, all here!

Encyclopedia of Monsters--Overflowing with information on all sorts of "mythical creatures and fabulous beasts"!

Medieval Art and Architecture--I came across Alison Stones' wonderful site while I was searching for a floor plan of a medieval castle.  You'll find that--and much more!--at this terrific site.  Wonderful for the fantasy or historical writer.

Also be sure to check out the website tools links and the writers links!