There are a lot of free online dictionaries available but these are a few of the best.
- WWWJDIC Best for J->E, useful collection of example sentences (which should be used with care as mistaken entries remain).
- goo辞書 Contains J->E, E->J and J-J dictionaries.
- ALC / 英辞郎 on the web J->E and E->J, useful example sentences but does not give readings or furigana.
- シソーラス J->J thesaurus. Note that it is limited to a certain number of searches (per day?)
Rather than try to list every dictionary on the web here is Jim Breen's Online Japanese Dictionaries and Glossaries page. Probably the most complete list available on the web.
Google is not a dictionary but it is one of the best ways to investigate words that you can't find in dictionaries.