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"!
Glossary of 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.