Subcategories:
Dictionaries 
1. Dictionary 
Free searchable English dictionary with over 200,000 word definitions and list of their synonyms.
http://www.wordthrill.com/
Free search access to a frequently updated database of words, terms, names, and acronyms. Features also an A-Z index.
http://www.free-online-dictionary.org/
Dictionary giving fun definitions of thousands of words and phrases.
http://www.fundict.com/
Searchable directory of glossaries and topical dictionaries.
http://www.glossarist.com/
A collection of terms, definitions and acronyms related to the art of soap making.
http://www.ccnphawaii.com/glossary.htm
Free searchable medical dictionary with many thousands of medical and health related terms.
http://www.medicaldictionaryweb.com/
Guide to slang heard around the world.
http://www.slanguage.com/
A quick reference for Filipino tagalog words and phrases.
http://www.tagalog-dictionary.com/
Online dictionary where words are defined as pictures for learning the exact names of objects. Differs from a regular dictionary in that the images replace the words. To learn by way of images with thematic pages that have concise and rigorous texts.
http://www.infovisual.info/
Synonyms dictionary listing more than 30,000 words with their synonyms. Browsable database of synonyms and related words.
http://www.synonyms.ca/
11. Wikiwords.org 
An open dictionary built by translators - A collaborative project to create a dictionary of all terms in all languages with definitions and example sentences.
http://www.wikiwords.org/
12. Word Definitions
Searchable online dictionary with over 160,000 word definitions.
http://www.worddefinitions.net/