"Ci vuole del sale!" http://ilgur.com/2007/03/07/lesson-101-understanding-verbs-volerci/
"Detta così" (and not "Detto così") http://www.wordreference.com/iten/detta%20cos%C3%AC
"Per strada" "In strada" "Nella strada" http://duolingo.com/#/comment/258578
As usual, it will take time to have a longer list. I will try to put here idioms and words that work in a pretty unnatural way for people studying Italian.
Still not so much stuff here, I must understand how to organise it. If you have any questions on idioms, please put them here!
This has great potential ! What I as a learner find difficult is to know which idiomatic sayings are in common usage and which are obscure and not worth the effort of learning. I fear that as most of us are here because we are learning Italian there may be few who can contribute so we may have to rely upon you quite heavily to provide the content. Also, idiomatic sayings could fill an entire book (indeed I have one entitled "2001 Italian and English Idioms" ISBN 0-8120-9030-6) so it is potentially a huge subject. For my own part one of my favourites is "In bocca al lupo" which I shall wish to you.
I tend to add stuff from the questions I answer to. There are many versions of "in bocca al lupo", but far way less polite. "In culo alla balena" and "Tra le palle del riccio".
When I was a kid I had my own way to say "In bocca al lupo", that was "Allupo", while my brother said "Inpoccollo". XD Please don't learn those two made up expressions please! :D