Lacunae reminded me of another one:
“I didn’t go to school to eat my lunch”. This means that you actually learned something in school, rather than just warming the chair there.
I learned a lot from my ESL students, because I’ll ask them to pick an idiom from their own language, translate it literally and then describe what it’s supposed to mean.
One that I use now is Russian, “the wolves of Tambov are your friends.” Tambov is the location of a famous prison in Russia, the wolves are the violent criminals. So to malign someone’s character, you can observe that the wolves of Tambov are his friends, implying that the guy is too sketchy to have normal friends.
Not from ESL, but a favorite that I picked up from a friend’s mother who is from the Midwest is used to describe any of those times when you have a random ache or cramp, like if you’ve been sitting in one place for too long. Then you can say “I have a hitch in my git-along.”
“You might as well piss in the Ocean” For something that technically helps, but is of such little volume that is is effectively useless.
Well, there’s the direct one “ya dejad de discutir el sexo de los ángeles,” “stop arguing about the sex of angels.”
Then you have, when two people are talking very animatedly but about nothing much, “aquí, resolviendo los problemas del mundo” (here, solving the world’s problems) or specific problems like “aquí, pacificando Israel” (here, pacifying Israel).
A conversation where neither part listens to the other is “un diálogo de besugos” (a conversation between two (fishes); I don’t know what are besugos called in English but the word also means moron), or “de dónde vienes, manzanas traigo” (where do you come from, apples I bring).
There are two whole books dedicated to direct translations of Spanish idioms into English; sadly, they didn’t add what the actual translations would be. If you’re interested, they’re called “speaking in silver” (literal translation of “hablando en plata” which means “speaking clearly”) and “from lost to the river” (“de perdidos al río,” best translated as “… so damn, let’s do it!” or “from the frying pan into the fire”).
One great phrase learned from Mississippi Elder Othar Turner is: “A Fail ain’t nothing but a Try.” He said it many times in talking to him, and, though I didn’t quite get it at first, it comes up often now in my mind in understanding life. Great sweet wisdom and encouragement.
I cut this thing twice and it’s still too short.
That’s about as obvious as balls on a tall dog.
There’s one in Chinese that, when literally translated, goes “Loud farts don’t smell, the really smelly ones are deadly silent”. This means if you want to have real impact, be deadly but silent… instead of blowing a lot of hot air.
My dad (Wisconsin) always said he had a hitch in his giddyup.
“That’ll go over like a fart in church”
“As quiet (or as loud) as a gnat’s fart”
“Looks like two riled cats in a sack” (regarding a large behind, usually on a woman)
Ahh, as nervous as a whore in church!
My dear old Grandaddy in Mississippi is sometimes very defeated by life in general or his health issues. Then he is in low cotton.
I love the Australian ones:
It was on for young and old! There’s about to be a helluva fight - “Jim told Sally she was a slut in front of her brother and his mates, and then it was on for young and old!”
Off like a bride’s nightie/bucket of prawns in the sun Something that is very quickly removed.
Two chances, Buckley’s and none (Can also be said as just “He’s got Buckley’s.”) No chance at all, Buckley was an explorer who died in the outback.
Rattle yer dags Move quickly (Dags being the dried sheep shit stuck to a sheep’s butt - it rattles when they run.)
Up and down (or on and off) like a whore’s knickers.
Two I’ve never entirely understood, All mouth and trousers and Fur coat and no knickers
“Hung around like Grant hung around Richmond.” (Tenacity)
From Arabic;
“He is a pilgrim and also sells prayer beads.” (He plays both sides of the game.)
Happy as a clam. (Short for “Happy as a clam at high tide,” because that is the only time a clam is safe and getting something to eat. )
A foolish person, according to my mother, doesn’t have the sense God gave little green apples.
Things are hopeless: You got two chances, slim and none. And Slim just left town.
Love the wolves one.
A “hitch in his git-along” is very useful to describe lots of things. If something is holding up a process; if someone seems to be having a lot of trouble getting something done; if something is mysteriously wrong with your car. I use it often.
One I never heard until I came to work for the City here is “turn your hat around”. This is said to someone in transit if you think they’re taking to long to get there. “Are you coming to fix this leak?” “Yeah I just passed Bubba’s house” “Well turn your hat around!”.
The full version is “Turn your hat around to make it look like you’re coming back.”
I know, cause my Grandaddy said it to me near daily until I left the state. 
It’s hotter than two rats fuckin’ in a wool sock.
I am still grateful to my parents for being Southerners for just this reason! Some of the more amusing and/or ineffable ones:
“Happy as a dead pig in the sun”- I really don’t quite get this one, but I like it.
“As independent as a hog on ice,” usually used to subtly disparage a young person being stubbornly autonomous but out-of-control, with little piggy legs skittering out in all directions at once.
“Fifteen monkeys fucking a football”- a more colorful way to describe a “clusterfuck” project: a whole lot of excitement going on, but the ball ain’t moving.
“Dumb as a sack of wet kittens”
“Ugly as a bucket of assholes”
Beware the pedant!
It doesn’t work because they’re talking about sows under that heading. It surely doesn’t matter how many nipples a boar has, although there might be another idiom in that there concept! 
Anyway, a few of the gems I like are a bit profane, natch; Eastern Arkansas is sometimes described as “flatter than a plate of piss”, and in the urine vein, when it rains real hard, it’s “like a cow pissing on a flat rock”.
Bonus missed the edit window special:
In the swine vein, when you’re in the money, you’re “cuttin a fat hog in the ass”, which is much more interesting than “high on the hog”…then there’s “low on the hog”, which, IMHO, is the most concise definition of scrapple in existence.
Having a shot of hard liquor, I’ve heard described by oldsters, is “something for my wife’s kidneys”. Yow.
“Not enough ________ to plug up an ant’s ass” - a minute quantity of something.
And read here on the Dope awhile ago, to describe a dullard trying to figure something out: “He’s turning it over like a monkey trying to fuck a football”.