Under the category of things that I love is the Online Etymology Dictionary. It tells you where words come from and when they were first used. With words that have several senses, it lists when the word first acquired each meaning.
I always have in mind that Bernard Cornwell said he tried to only use words that would have been in use in the early 19th century when he was writing the Sharpe novels. Large parts of my work in progress are set in the 18th century and if a word sounds quite modern to me, then I'll check it.
Besides, I just love words :-)