IdiomDictionary has a killer domain name, because that's exactly what it is: a dictionary for idioms. I have no idea how much the domain cost, but it appears to be quite clean of advertising.
The site claims to explain the meaning of over 5,000 idioms. It does not seem to use an "established" dictionary for the definitions, but the explanations seem to be well written, concise, and authoritative.
Many dictionaries explain idioms, and OneLook lets you search for the idiom directly. But one unique IdiomDictonary feature is the "Nearby Terms" list; it's like a thesaurus just for idioms. So, let's say you have an idiom in mind, but it's a bit too trite or offensive, and you want to go for another, similar idiom. A regular dictionary won't help you out there, but IdiomDictionary will.