The Phrase Finder has an impressive collection of phrases, idioms and expressions commonly used in English. It is almost like an online dictionary of English expressions, but with relatively lower database. However, I think the collection is appropriate for intermediate level learners. Also, what separates it from regular dictionaries and makes it more interesting is its Meanings and Origins section. I really like the way it groups the commonly used expressions under different categories. These can be helpful in introducing learners to expressions based on a common theme.
I would also encourage my students to sign up for the free A Phrase A Week service available on this site. (Much like myself!)While some of the phrases may be of a little advanced level, but it will give them a sense of how words can have metaphoric meaning in a language.
