Raining cats and dogs
Unfortunately it's one of these that students often learn and remember, even though it's not commonly employed by English speakers, who tend to say things like "It's chucking/throwing it down!". When it is used by English speakers, it's usually as a joke, which is a nuance that students may completely miss. There are plenty better examples of English idioms 
Ideally this would be backed up by corpus-based linguistics.