Mythology

The Encylopedia Mythica provides information about mythology, folklore, and religion. 

http://www.pantheon.org/

Visuwords

Here's a graphical way of looking at the relationships between words. 

Reference Books

Seven reference resources for writers. 

http://www.dailywritingtips.com/7-reference-resources-for-writers-and-editors/

Online Reference Desk

Hundred of links to fast fact sites, such as almanacs, magazines, organizations, etc.

http://www.refdesk.com/fastfact.html

Online Reference Sources

Seven websites to help you do research for your nonfiction or fiction.

http://www.dailywritingtips.com/7-great-online-research-resources-for-writers/

Reverse Dictionary

When you can't think of a word, describe it in this reverse dictionary to find the word.
http://www.onelook.com/reverse-dictionary.shtml

Slang

American, English, and urban slang, including words used with social media.
http://onlineslangdictionary.com/

Dictionary Searches

One look lets you search multiple dictionaries using wildcards, abbreviations, phrases, and more.
http://www.onelook.com/

Historical Words

Want to know whether the word you're thinking of was in use at the time of your historical novel/book?
http://www.etymonline.com/index.php

Holiday List

Looking for an unusual holiday to celebrate in your story?
http://www.brownielocks.com/

Book Publishing Glossary

Agent Nathan Bransford defines many of the common terms used in the publishing world
http://blog.nathanbransford.com/2009/08/book-publishing-glossary.html

Resources for Children's Writers

Links to many internet sites of interest to children's book writiers.
http://resourcesforchildrenswriters.blogspot.com/

Reference Books Online

Search reference books such as Bartlett's Familiar Quotations, Gray's Anatomy, and Oxford Shakespeare.
http://bartleby.com/

Acronyms

List of commonly used acronyms in the children's writing world.
http://www.scottrayrandall.com/childrens_writing_acronyms.html

English Usage

Online source for grammar, punctuation, and capitalization rules.
http://grammarbook.com/

Librarians' Internet Index

An organized listing of internet websites you can trust.
http://www.lii.org/

Ask a Librarian

Online reference service from the Library of Congress.
http://www.loc.gov/rr/askalib/