Here are some sites with free online etymology:
- Online Etymology Dictionary (www.etymonline.com) - a unique source of over 26 thousand detailed and distinct etymology entries
- Wiktionary, The Free Dictionary (www.etymonline.com) - a collaboratively produced dictionary with over 2.3 million English definitions from over 400 languages -- not all entries have etymologies, but many do
- Oxford Dictionaries (oxforddictionaries.com) - bills itself as the source of "the most trusted dictionaries" and includes over 107 thousand distinct entries, many with eymology information
- Merriam-Webster Online Dictionary (www.merriam-webster.com) - the online edition of this leading American dictionary has over 113 thousand entries, many with eymology information including when a term was first used
- YourDictionary.com - hosts Webster's New World College Dictionary and American Heritage Dictionary of the English Language, 4th edition for a total of over 135 thousand entry pages and over 92 thousand audio pronunciations generated by speech synthesizer. Both sources have etymology information
- MSN Encarta U.S. English Dictionary (encarta.msn.com) - this site appears to no longer be actively maintained but it's none the less a useful resource with over 105 thousand entries and many of them with brief etymological information
But my personal favorite site is
memidex.com which brings all the above sources and their etymologies together.