Merriam-Webster's Learner's Dictionary
This online dictionary is geared toward ESL and EFL learners. It allows you to search a word and not only receive a definition, but also see multiple meanings and the use of the word in a variety of sentences; a recorded pronunciation; and a list of common idioms. The site also offers pronunciation practice with Perfect Pronunciation Exercises, a list of common words to know, a subscription to Word of the Day and the ability to create your own vocabulary flashcards with My Learners Dictionary (must set up a free account).
Ex. When asked to search the word 'cat', the site pulled up two definitions 1) a small animal related to lions and tigers and 2) old fashioned use: a man as in "you crazy cat". It gave 9 idiomatic expressions (i.e. "let the cat out of the bag"). It also provided a picture of several types of cats.
In addition, there is a blog link located on the website, which allows you to ask questions about vocabulary or grammar. You must set up a free account to access the blog portion to make comments or post a question.
It's a wonderful online dictionary for ELLs that gives them a simple and precise definition with a variety of examples and sentences. The exposure to multiple meanings helps develop a deeper knowledge of the vocabulary word. Additionally, the idioms also help students see how particular words are used. It's a great online tool that allows students to access vocabulary words outside the classroom to supplement any vocabulary lesson, curiosity or word research.