Here are some links to English-to-English Dictionaries Online.
http://dictionary.cambridge.org British English. Doesn't have voice pronunciation, but IPA transcription is even better.
Geona - Used to use it in the past. Main drawback is a lack of IPA transcription.
WordNet Search - Fast lookup, provides good explanation by related words (synonyms, antonyms, etc).
Dictionary.com - Popular dictionary with pronunciation explained. Slightly overloaded with ads.
Sometimes it's just handy to use Google's Definition Tool: type "define: keyword" in the search box to get all the explanations of the keyword found all over the Web.
Merriam-Webster Online - Free On-Line Dictionary, Thesaurus and many other learning things like games, word-of-the-day, etc. Some of them are not free.
Encarta - Dictionary from Microsoft. US English with voice pronunciation.
