I like LEO: http://dict.leo.org/ende/index_en.html

"Pons" is one of the bigger publishing houses for dictionaries in Germany, and their free website http://de.pons.com/ is quite good. It also includes verb patterns. It might include a few idioms, but in general I don't know of any dictionary that actually translates idioms. E.g., when I need to look up modern English idioms, I usually use an English-English site like http://www.urbandictionary.com/ Googling for "deutsche idiome" finds a number of German-German sites, e.g. http://www.redensarten-index.de, but I haven't used them, so I can't say how good they are. For natural translation of expressions, http://www.linguee.de/ (correlated public texts in several languages) is very helpful.

There are a lot of idioms on http://www.dict.cc/.

For German translations of English Idioms I'd recommend: http://context.reverso.net/übersetzung http://www.phrasen.com/ For context sentences for idioms I suggest this site: http://en.bab.la/dictionary/english-german/