Idioms can be confusing. Is a Monster in the closet literally... or figuratively? And what does it mean figuratievly anyways?
My students are actually pretty good at these (YAY for one thing I don't need to review for this Summer's TOEFL course) but what I really like about this site is the ability to find idioms in other languages.
This way you can give students an example of something that is not literal in their language (compared to a direct translation of the english idiom).
Yes, yes, I know I know avoid translations when possible, but this is something I have found that really helps students understand the concept behind idioms rather than just memorizing words which don't seem to make sense.

Let me know how you teach idioms or what you think of the site! (see more from Carissa here: http://eslcarissa.blogspot.mx/)