Italy's language academy answering usage questions — the final word on 'is this correct?'
Details
The consulenza linguistica archive holds hundreds of expert answers to exactly the questions advanced learners get stuck on: which auxiliary, whether a form is regional, whether a new anglicism is acceptable. Everything is in Italian and written for an educated native audience, so it is a C1-ish read. Free, and you can submit your own question, though only recurring or generally interesting ones get answered.