What are comparative strong points of online dictionaries and other useful reference sites?

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There are two sites that I most commonly use as a first stop when looking up words online:

• onelook, the reasons are that, for example, looking up a word "dictionary" gives not only quick and short definition (from macmillan), but also links to 44 other dictionaries and to etymonline

• wordnik, the reasons for this one are that it gives you immediately on the results page definitions and examples from a few dictionaries that you can compare, however it does not give links directly (compare a lookup for "dictionary" - it gives definitions from four dictionaries, reformatted to consistent look and some examples)

(Both of these are aggregating from different sources, but similiar reasons could be given for specific dictionary sites).

I would like to find out which other dictionaries or sites people are using as the first stop (regardless if they are aggregators or specific dictionaries) and why (in a sense of how or when they would be best used)?

Please one site per answer.

Solution 1:

Wiktionary

Why?

• Lists common misspellings, like compatability and noone.
• Clean layout
• No abbreviations to decode
• Simple URLs. For example, http://en.wiktionary.org/wiki/compatability
• Multilingual (even though it is very sketchy in many non-English languages)
• In-place correction of any mistakes in the dictionary (it is a wiki)
• Comprehensive coverage of initialisms and Internet terms and slang. Examples: BTW and Google-fu.

Solution 2:

The Oxford English Dictionary is indispensable for any serious inquiry about English, but it requires subscription. Those in the UK can have a free subscription through their county libraries. It provides authoritative and unmatched information on a word’s etymology, pronunciation and meaning. Definitions are supported by citations showing how words have been used since their first appearance.

Solution 3:

To find the meaning of English words, for synonyms and discovering new English words, I use WordReference. Mainly because it is the one I've been told about, and that its features are the one I need. It has a very simple and efficient interface.