Online dictionaries

As an English-majored student, we often need to rely on the English dictionaries to look up words. And the most common source is the on-line English dictionary.
Here are the dictionaries with links that my teacher recommended:
(1) Cambridge Dictionaries Online: http://dictionary.cambridge.org/
(2) Oxford Advanced Learner's Dictionary: http://www.oup.com/elt/oald/
(3) Merriam-Webster Online: http://www.merriam-webster.com/
(4) Dictionary.com: http://dictionary.reference.com/
(5) OneLook Dictionary Search: http://www.onelook.com
(6) Acronym Finder: Find out what those acronyms and abbreviations stand for: http://www.acronymfinder.com/
(7) Yahoo!奇摩迷你筆: http://tw.mini.yahoo.com/

The most common tool I use right now is the “Yahoo Mini Pen”. It is quite convenient to look up the words that you don’t know, especially when you are reading the articles online. All you have to do is using the pen to underline the words that you are not familiar with. The meanings of the words will show up right away without any copy or retype on the other online dictionaries. Besides, when you want to look up equivalent words, you don’t need to open another window. However, Yahoo Mini Pen only provides simple explanation, synonyms and antonyms of the words and example sentences. So it might not be complete and detailed as you think.
The dictionaries that I suggest are “Cambridge Dictionaries Online” and “Oxford Advanced Learner’s Dictionary” which are the classic dictionaries that we know. Both dictionaries provide complete explanations of the certain word as well as example sentences and usage. You might have personal preference for dictionaries. But to me, when I look up the meanings of a word, I would like to use “Cambridge Dictionaries Online” for its clear format. When I want to know the meaning of an idiom, I would choose “Oxford Advanced Learner’s Dictionary” instead.
No matter which dictionaries you use, you have to be aware that these dictionaries are “online dictionaries”, which means that you need to get a computer and an internet access.