Here's a list of some of my favourite dictionaries online (five to be precise).
  1. etymonline.com - great dictionary for finding out where certain words come from.  I'm always checking it as I tend to think about those kind of sad things!
  2. myetymology.com - similar to etymonline but may have a few more words I find.  The descriptions are not as full though and it's laid out more like a normal dictionary entry.
  3. wordreference.com - very useful for translating words quickly.  It has a great feature which you can use to quickly select between languages if you're not sure what language a word comes from.
  4. wordweb.info  - don't know how I found this one.  I think I was looking to download a dictionary in the days before I had broadband so I downloaded this one and it's very good.  It gives you words close in spelling/synonyms/types or you can look it up online as well
  5. yourdictionary.com  - I've used the forum of this site more than the actual dictionary though the dictionary does look good too.  You can find discussions on the forum about lot of anorak type English language stuff so don't worry.
  6. I know I said 5 but this one deserves an honourable mention:  onelook.com  - this gives you the chance to look up a word in a number of different types of dictionaries which is very beneficial and also can lead you to more information than you bargained for.

So there you have it.  That's my list of Top Five Dictionaries online (plus one extra just in case).  Hope these help you whatever you might be looking up.  
Take care,  


Wesley
Real English Online
www.realenglishonline.com
http://realenglish0nline.blogspot.com/