The list so far:
“He’s not the sharpest tool in the shed”
“He’s not the brightest penny in the fountain” (courtesy of The Simpsons)
“He’s two fries short of a Happy Meal”
“He posts to the wrong forum.”
He’s a couple of beers short of a picnic.
A few sandwiches short of a circus.
Wow! Thanks for the link Aglarond, that should do it.
Sorry about the misplaced thread, manhattan. Hope you didn’t hurt your finger redirecting it.
He…
…is not the brightest crayon in the box.
…is not the sharpest pencil in the box.
…is a few bricks short of a hod.
…wouldn’t notice the different between light and dark.
…has the intelligence of a half-baked loaf of rye bread.
…is a 10-watt lightbulb trying to outshine his relatives in Las Vegas.
…is smarter than a dead rock.
Mercifully spared the ravages of intelligence.
(That’s not quite right, but something like that…from Blackadder I think.)
This thread reminded me of this employee eval. Kind of off topic, but has some good ones in it.
Employee Evaluation
Name: ________________________ Date: __________________
Position: ___________________
==================================================
Knowledge:
The Son of a Bitch Really Knows His Shit!
Knows Just Enough To Be Dangerous.
Only Has Half a Brain and is Dangerous.
Fucking Brain Damaged, His Coffee Cup Has a Higher IQ.
Accuracy:
Does Excellent Work If Not Pre-occupied With Pussy.
Pretty Good; Only Occasionally Blows it Out His Ass.
Has to Take His Shoes Off To Count Higher Than Ten.
Couldn’t Count His Balls And Get The Same Number Twice.
Attitude:
Extremely Cooperative (If You Kiss His Ass Daily).
Brown Noser In Good Standing.
Often Pisses Off Co-Workers, Thinks He Owns the Damn Place
Doesn’t Give A Shit, Never Did, Never Will.
Reliability:
Really A Dependable CockSucker.
Works So Hard That He Has To Take An Extra Day Off Each Week.
Can Rely On Him To Be The First One Out The Door.
Totally Fucking Worthless.
Appearance:
Extremely Neat, Even Combs His Pubic Hair.
Looks Great On His Days Off.
Dirty, Filthy, Smelly Son Of a Bitch.
Flies Leave Fresh Dog Shit To Follow Him.
Performance:
Goes Like A Son Of A Bitch…If There’s Money In It For Him.
Does All Kinds of Good Shit At Evaluation Time.
Works Well After An Enema.
Couldn’t Do Less If He Were In A Coma.
Leadership:
Carries A Chain saw And Gets Good Results.
Macho Attitude, Commands Total Disgust.
Dog Fasted For Three Day Last Time He Brought Home Pork Chops.
Mother Teresa Told Him to Get Fucked.
I UNDERSTAND THAT I HAVE BEEN COUNSELED AND UNDERSTAND MY RIGHTS UNDER THE PRIVACY ACT OF 1974. I FURTHER ACKNOWLEDGE THAT I AM FUCKED UP AND WORTHLESS AS A FOOTBALL BAT, AND I WILL MAKE SOME KIND OF ATTEMPT TO CORRECT MY OBVIOUS DEFICIENCES.
Employee Signature (If He Can Write)
…not the brightest bulb on the Christmas tree.
…not the sharpest knife in the drawer.
…no rocket surgeon.
…couldn’t pour water out of a boot if the directions were written on the bottom.
…couldn’t organize a rock fight at a quarry.
He’s about as sharp as a bag of wet gerbils.
The one that never fails to amuse me (don’t know why) is:
Dumb as a bag of hammers.
My dad’s favorites:
Dumber than a coalbucket.
Couldn’t find your butt with both hands.
My personal:
Daft as a brush.
Not one is clever, but they amuse the heck out of me.
A taco short of a combination plate.
He’d make a fine manager.
Mauvaise stole mine. 
Though I always say “Dumb as a box of hammers.”
That list that Aglarond linked was fun. At first I was puzzled by “collects cards for Craig,” but I decided I really like that one, and plan to use it endlessly.
Sharp as a marble.
Bright as a cavern.
Dain Bramaged.
Couldn’t find his own ass with both hands and a flashlight. (that last bit’s important)
Doesn’t have the brains God gave a duck.
Pushes doors marked “pull.”
Not smart enough for government work.
“Suppose you were an idiot. And suppose you were a member of Congress. But I repeat myself.”
–Mark Twain
I’ve got more “ugly” ones myself, such as:
Fell out of the ugly tree and hit every branch on the way down.
Got a serious whomping from the ugly stick.