Which are the best online dictionaries for learners of English?

Here's a link to another blog which you may find helpful if you're not sure which online dictionaries to use:

http://shar.es/AHM3h

I would add that I like http://dictionary.reference.com/ because the definitions are clear and easy to understand, you can click to hear the pronunciation of the words and because examples of the usage of the words are given, including use in idioms.

As always, feel free to ask questions or add comments here!