It's raining cats and dogs
similar idioms in other languages, according to wikipedia.
"Crowbars" (Bosnian)
"Old women with clubs" (Afrikaans)
"kittens" (Dutch)
"Snakes and lizards" (portuguese)
"chair legs" (Greek)
"shoemakers' apprentices" (Danish)
"she-trolls" (Norwegian)
"husbands" (Spanish)
♫ It's raining men / Who are husbands ♪
Thanks to Michael Yin for suggesting this one.