Translators first - everyone knows Babelfish, but I have a few others I like.
This one, here, is not entirely serious but with a little editing I've found it really useful for writing accents - Scouse and Scots especially.
Want to know how to swear in Swahili? Or Russian, or French? Try here. Also a fairly entertaining quotes site.
Now, random writing research.
If you write yaoi, I would strongly recommend Minotaur's site. I warn you, it's illustrated and is definitely not for kids, but it will help you get your slash biologically and socially accurate. Although there has been a side-effect that since reading this site I've become more annoyed by writers that obviously don't do their research.
Every writer who touches on other cultures should have Encyclopedia Mythica in their bookmarks. No question.
Also, there is of course the Writers' and Artists' Yearbook. Good to have online, especially since the books themselves are expensive to buy.
I warn you, before clicking this link - if you go into Everything2, it can be hours before you come out. But you will almost certainly have found out something new.
And now some specifics, for fantasy writers:
Phases of the moon, 2001-2025. Very useful for werewolf stories.
Tarotpedia - it's a wiki about Tarot cards, their meanings and history. What more can I say?
Hope this has been helpful!
Cross-posted to my journal.