Probably my most visited site is the Online Etymology Dictionary. Seems like a silly link, but I'm in love with words and their meanings and their origins. I think that knowing those things helps me be a better writer. It also helps me come up with names and names of places. I spend way too much time on this site, and for that I blame my old Medieval Lit professor, Dr. Mary Faraci.
Here's the International Swearing and Cursing Dictionary. As my agent knows, I tend to have a potty mouth. Maybe one day I'll write a post on why I think Americans are too uptight about profanity, but swears in other languages help me get around the issue a little.
To go along with my love of words, I also like hiding meanings in my names and places. So the Internet Anagram Server is used frequently.
Creating original and vibrant dialog is tough for any writer. Use slang that's too current and you risk it being out of fashion by the time the book comes out, too old and you sound out of touch. I use the Urban Dictionary when I'm looking for just the right word. I tend to favor British slang because it can be funny but it's not well known here in America. That gives it the impression of new slang. I've even added some words of my own.
Here's an article about 10 Mistakes Writers Make. I refer to this frequently. Even with a book deal, I still feel like an amateur. I don't agree with every bit of this but it's still worth reading.
Wordnik is a relatively new site, but one I adore. It's more than just a dictionary. It's like a word extraveganza. It gives you definitions from multiple sources, grammar info, etymology, usage, even recent uses in Twitter. Pretty awesome.
Finally for today, I've wasted more time on this site in the last month than I care to admit. Atlas Obscura is a site that lists strange and unusual places around the world. It's an awesome site for inspiration.
Strange salad alien via Mopo