http://www.usingenglish.com/reference/idioms/

Here are a couple of tricks I use: 1) Go to http://news.google.com and put your idiom in quotation marks when you search. This way, you get a proper real-life example, in context. Do a search with the idiom in past tense as well. 2) I've found that the forums on http://www.phrases.org.uk tend to have the more sensible explanations of idioms. Do your search there as well. Some idioms websites don't really give a helpful explanation, and in some cases the background story is completely made up (and ridiculous).

http://www.idiomconnection.com/