For me, it’s “tits on a boar hog”.
In keeping with the porcine theme, how about:
“Useless as trying to teach a pig to sing”
or
“Useless as trying to make a silk purse out of a sow’s ear”
BCS 
Decaffeinated coffee
Yahoo’s Fantasy Football predictions
Non alcoholic beer
My old broken Nikon F2AS finder (can’t see thru it, can’t meter with it, gone digital now anyways)
An old stool sample of mine I found
Wait…what?!
Patches O’Houlihan: “You’re all about as useful as a cock-flavored lollipop.”
A bunch of old keys.
Nah - lets the condensation drip onto the table. However, if you’ve got a dozen or so of them and a hot glue gun they make a nifty Christmas decoration.
My contribution: bottle caps. My daughter, for reasons that I cannot comprehend, saves caps from bottles she gets when we’re eating out. Needless to say, I throw them out as soon as she’s out of eyesight.
You forgot “gas on the ground”
I disagree; non-alcoholic beer has allowed my father to get the satisfaction of a beer after a day’s work over the past 20 years without having to deal with the alcohol, which he had previously proven he had a severe problem with. He’s a much better man now than before!
Yes. I also tie the nylons with runs in a knot, and throw them back in the drawer. 
[SIZE=“1”]and put that carton with a quarter-inch of milk back in the fridge.[/SIZE]
Hell, yes. We tried that. Once. the cat quickly let us know what she thought about that. 
Cool.
For me, tho …
Phone books are quickly becoming doorstops for me. Last time I actually opened one, I was looking for an oil change coupon. (Found a better one online.)
Well, in a sense the harness proved useful - all you had to do to get the cat to vanish into thin air, was to lay one’s hand on it.
Yeah, I can’t stand the stuff either. We have 4 left in the back of the fridge from the last time he was here, and even if I really, really want a beer, I won’t touch the 0.5! Dad says it tastes like shit, but he’d rather have that than nothing, so good for him.
on the theoretical
a screen door on a submarine
on the practical
pens without ink can use the tube for passageways for solids, liquids or gases. a fabricator considers them raw materials that should be kept out of the writing instrument storage area.
- A cell phone (not a smart phone) with no signal.
- Flour and sugar but no butter or eggs.
- Flat soda.
- Snooki
For safety’s sake, you’re best off pitching that wand as soon as you open the shadow and using a good brush for application, so that oil & bacteria from your skin aren’t left in the compact, incubating and contaminating your makeup.