Etymology http://www.etymonline.com/

Root word dictionary (Greek & Latin)

Name origins http://www.behindthename.com/

Word origins http://oxforddictionaries.com/words/word-origins

Merriam-Webster Dictionary http://www.merriam-webster.com/ 

Cambridge Dictionary & Thesaurus http://dictionary.cambridge.org/us/ 

Dictionary.com http://dictionary.reference.com/

One Look Dictionary http://www.onelook.com/

Dictionary of Medieval Names http://dmnes.org/about


Allwords.com   Has the traditional dictionary feature along with specialized word searches and translations.
Bartleby.com   A site worth the time to explore it. Contains a myriad of features for writers that include a dictionary, a thesausus, an encyclopedia, an English usage book, and a style book.
Dictionary.com   Contains a dictionary, thesausus, and encyclopedia features amoung others.
Encarta.msn.com   Includes a dictionary, thesauses, translations, and encyclopedia features.
Merriam-Webster.com   Can use the dictionary, thesaurus, Spanish-English translator, or medical search feature by word or phrase.
Wordsmyth.net   Users will need to register to use some of the features on the site; however, registering is not required to look up an exact definition.
YourDictionary.com   In addition to dictionary and thesaurus features, this website provides specialized dictionaries and word games.   

Other Dictionaries
21 Century teaching terms dictionary