AskOxford- Compact Oxford English Dictionary
The Compact Oxford English Dictionary is also online. It is also published by Oxford University Press, but is a much smaller and less comprehensive dictionary. It contains 145,000 definitions, instead of the more than 500,000 definitions of the OED. However, it allows the ability to search for free to anyone who accesses the website. As well as the Compact Oxford English Dictionary, it allows you to look up a word in the Oxford Learners Dictionary.
It also has a dialog box for AskOxford which can be downloaded to your website, allowing a word to entered, which is then defined by the Compact Oxford English Dictionary. Their definition is not as complete as that of many dictionaries, but includes the parts of speech and meanings of the word. However, they also include definitions from other sources in their list of links, so it is possible to see what several different dictionaries give as the definition of the same word. It does not provide pronunciation, the etymology, or the quotations using the word found in the OED.
The AskOxford site has numerous resources for teachers and students of English as a Foreign Language. You can find work sheets with exercises for English comprehension and news articles in English, with questions to test your comprehension of your reading. There are also online practice tests for English exams.
There is also a webpage on the site where several Applied Linguistics books by prominent authors can be downloaded for free.
You can also purchase the Compact Oxford English Dictionary for about $23 USD or £16.99 , and it does provide pronunciation and language of origin in the printed volume.
Cambridge Advanced Learners Dictionary is one of several free online dictionaries affiliated with Cambridge University Press. These dictionaries are particularly useful to learners of English as a Second language. They do not provide the etymology of the words they define, because they concentrate their resources on contemporary English usage. However, you can toggle on the phonetics to see how to pronounce a word.
The Cambridge site also provides an additional list of dictionaries you can search in, if you do not want to use the Cambridge Advanced Learner's Dictionary. These include American English, Idioms, and Phrasal Verbs, such as 'go' or 'be', and an English French dictionary. These dictionaries are also available for sale on their site, most with both a hard copy and a CD of the dictionary.
Cambridge Dictionaries Online also provides a number of free resources for English Language teachers, and learners of English as a second language. They also publish a series of books, Cambridge English for... Engineering, Nursing, and other specialized professions where a specific English vocabulary is needed.
Cambridge University Press also has a number of RSS feeds earmarked for English Language Teaching resources.
The Cambridge Advanced Learners Dictionary is available in paperback with CD for $34 USD, or £22 + VAT.