Thanks to an English teacher who is now following this blog, Joan Rubies, I have had the opportunity to know this wonderful tool. I also recommend to visit his blog, full of good ideas and tools for learning English.
Memidex is a free online dictionary and thesaurus.
Memidex reaches a new total of 12.5 million external reference links and 1.8 million etymology references.
Each audio reference has a button to play the audio directly from the Memidex web page.