Online Dictionaries

• Dictionary.com - This online dictionary is the one I use most frequently. It provides ample definitions and expressions, and Thesaurus is greatly helpful for ESL writers. There is also Translator for over 50 langauges from word level to sentence level, but, I found the work of translation not reliable.
• Merriam-Webster Online - This one also provides dictoinary and thesaursus, similar to Dictionary.com, I would say.
• Cambridge Dictionaries Online - This online dictionary provides both US and British pronunciation samples which a learner can actually listen to. It also has several sub-dictionaries such as Advanced Learner's, Learner's, American English, Idioms, and Phrasal Verbs, among which I like the last two most. They provide ample explanations and examples of idioms of a word, which is really helpful for English learners in that they generally have a weakness with idioms or formulaic expressions.
• Oxford Dictionaries Online - This online dictionary offers two versions of dictionaries, US English and World English, and you can choose one of the two options at the front page. On the top menu, right by Dictionary, there is the menu of "Better Writing" which offers useful tips on grammar, spelling, pronunciation, and writing. Especially, I find the section of "Improve your English" very helpful; that is, it works on nuances (bored by, of, and with), easily confused words (ex. affect and effect), and a list of different British and American terms (ex. dustbin and garage can)
• MSN Encarta - This one also provide both US and World English dictionaries as well as Thesaurus and Translations (which is basically English-other languages dictionaries). These dictionaries are only good for one-word lookup since they have limited entries for idioms/expressions and they do not provide many sentence examples, some of words and expressions only with definitions.
• Urbandictionary.com - This is a slang dictionary, made and constantly edited by users, so I would call it a wiki-dictionary. Since due to the Internet, English learners access to "authentic" expressions which are sometimes not found in traditional dictionaries, I have found this online dictionary very useful and helpful.
• google.com - This search engine is useful for ESL writers since it provides enormous databases including "language corpora," and by online searching, "a student can find which words in a language tend to go together, a phenomenon called collocation" (Horwitz, 2008, p. 144). When you are not certain about a certain English expression or idiom, you can search it through google.com and comparing how many results are found, you can decide which one is more widely used.

Teachers of ESL/EFL classes at all levels should explicitly teach how to use online dictionaries, promoting autonomous learning both inside and outside of the classroom and both current and future semesters. ESL writing teachers should also teach how to use thesauruses to find antonyms and synonyms. The last, but not the least, resource which teachers should teach is how to use online search engines which scan through diverse texts so that learners are able to check actual usages of English.