I can't say enough about Terragen, not on this page, anyway, so check out the page I've devoted to it.
If you wrote a romantic love scene, complete with full moon, set in the Pacific Northwest Aug 17, 1884 at 8 p.m., you'd be wrong. There was no moon that night. Here's a nifty virtual moon phase site so you won't make that mistake.
Need to convert imperial measure to metric? Here's a good place to start.
One of the handiest tools for an author concerned with getting dates right is a perpetual calendar. Here's a really good one.
Writing a nautical opus? Here's a handy glossary to help make your language salty and accurate.
Who hasn't wanted to write a book about pyrates? This site can help you sail a safe course, matey.
A good search site is Serebella. Easy to use and includes links to a few good writers.
Here's a list of words commonly confused by writers.
Need to know how many Euros a dollar is worth? Here's a site with a nice conversion engine for popular currencies.
What time is it in French Guyana? Find out at the world time server, and pick up a free utility to synchronize your computer with an atomic clock. There's even an event planner that matches times in any two locations in the world.