I have written before about the range of dictionaries and other vocabulary aids available online, but that was some time ago. Here is an update with three sites I have recently revisited and enjoyed.
http://www.rhymezone.com/ is great fun. Put in a word and find words that sound similar but not only that: play with the options in the drop down box. My favoourite is to search in Shakespeare. Students could get a lot out of these options also.
http://www.wordsmyth.net/ is self-defined as a dictionary made for education. It allows you to search for meanings in Beginner’s, Children’s or Advanced dictionaries. It is also worth checking out the Quizzes it allows you to generate. A useful site and one being developed to return to it in the future.
http://www.onelook.com/ is almost a dictionary of dictionaries. Search for a word here and it will find the links to dictionaries that define that word. That may sound unnecessary but it can actually be very interesting.