With so many possibilities of online dictionaries, it's simply impossible to write a blog post that is not extremely limited, so the following tips are based on my own personal opinion and more ideas, tips and links are more than welcome!
What I like about this one is that fact that it gives you the definitions of up to 10 different online dictionaries on the same page. Like most online dictionaries, you can also hear the words and see phonemic transcriptions. With the same style, you can also access thesaurus.com.
This online dictionary gives you a beautiful visual diagram of words definitions and their relations, like synonyms and antonyms, collocations, related words, derivation and much more (not to mention how cool it is to see the diagram being formed). With the same approach, there is also Lexipedia and Visual Thesaurus.
A very popular and easy-to-use speaking dictionary. If like this one, you might also enjoy Oddcast Text-to-Speech tool and Kako Messenger Text-to-Song tool (this last one is really funny).
Finally, here are two dictionaries that can be easily embed to your website, blog or wiki. Accessing their websites you can get the html code and the dictionaries will look like this: