Inspired by Usless magic items:
Thermonuclear hand grenade: its blast radius is far greater than anyone can throw it.
Raygun of disintigration: pull the trigger and it disitintigrates.
Solar-powered flashlight.
Parachute that opens on impact.
Power-line Networking
There really is a solar-powered flashlight.
The battery-operated battery changer that’s only good for changing its own batteries.
Hamburger Earmuffs
but you have to get the pickle matrix right.
Check here
[sub] Is this “our” Barking Spider?[/sub]
DUCK DODGERS: “Ha haaaa! Now, I’m going to shoot you with my dithintegrating pithtol! And brother, when it dithintegrateth, it dithintegrateth!”
[The Duck pulls the trigger. The pistol crumbles away into dust.]
DUCK DODGERS: “Well, whadaya know? It dithintegrated.”
Hmmm …
Hey, don’t knock it. It keeps the fish out.
The rechargable battery charger.
I’m shocked by Barking Spider’s list of useless inventions! Shocked, I tell you!
The list has “electric dog polisher” and “fur sink”, but not “gasoline-powered turtleneck sweater”! What would Steve Martin say?
There was a nuclear hand grenade in the movie, Voyage to the Bottom of the Sea.
How about see-through window blinds?
Braille instructions on drive-through ATM’s (yes, I know, but still…)
Fireproof charcoal (fun at parties!)
Biodegradable cars!
Underwater matches
Underwater barbecue grill
Wind-up CD/DVD player
Camouflage prison uniforms
Wooden fireplace
Glass crash helmet
Now can you tell me the origin of the other one?
Okay, I don’t get why Caffeine Free Diet Coke is a useless invention? What’s the ironic twist to being both sugar & caffeine free?
Well, why would anyone want to endure drinking Coca-Cola if they didn’t get a sugar high or a caffeine rush out of it? 
The Onion once had an article announcing that the inventor of O’Douls Non-Alcoholic Beer had just invented a non-adhesive glue, and was working on his next invention: flame-retardant gasoline
Coca-Cola is nasty enough as it is, without sugar or caffeine it is pointless to drink it.
Obviously, you don’t know somebody with a LADA
I’m not sure if you’re talking about simply networking power lines or communication networks over power lines (like this), but in either case, why would it be useless?