IMHO it’s, hands down, the cell phone.
Sure, why would we want to contact people any time we want, regardless of where we are?
I say it’s the pet rock.
We already had phones, both landline and public. Just who is so important they need to be reached every minute of the day? In emergency situations they worked just as well. And there was MUCH LESS identity theft.
I’d say the banana slicer. You can only use it for bananas that curve right, so right there it’s useless for half of all bananas.
Reported for forum change.
Stolen from a comedian: “A phone is just a rarely used app on my iPhone”
The cell phone (well, the smart phone more specifically) is the single most revolutionary invention of the last 30 years. Calling it useless is… strange.
I have to say the one that makes me struggle to understand it the most is those “crack the eggs into the plastic container and then hard boil them” gadgets. Eggs already come with a perfect egg-shaped boiling container. But I still see those things in the grocery store so somebody must be buying them.
The fact that we have 24/7 access to a searchable information system which is orders of magnitude larger that the Library of Congress is amazing. The fact that we carry in our pocket a computer that connects to this information is just short of a miracle. Oh, and also we can contact emergency services without looking for a phone booth. To call that “useless” is the strangest usage of the word “useless” I have ever seen. It’s so far away from the truth that it makes me wonder if the OP is trolling us.
There’s an entire website dedicated to a daily blog of terrible awful overpriced useless things. http://theworstthingsforsale.com . The October 31st 2018 entry is a jar of caffeinated maple syrup for $45. Personally, I’m torn between voting for the War On Drugs or a book that embodies the stupidity of the WoD: Latawnya, the Naughty Horse, Learns to Say “No” to Drugs by Sylvia Scott Gibson, which is selling on Amazon right now for $119, which is about $120 more than what I think it’s worth.
But for huge swathes of the globe landlines and public phones just didn’t exist, or were far to expensive or infrastructure dependent to install. So cellphones represented the first steps to an interconnected society. A definite benefit in my opinion.
Okay kiddos…since I specified “the last 50 years” I’m wondering if any of you were even born or remember a reality that wasn’t impeded by some assine idea of a sci-fi future.
As for the egg things, they were developed for those who have trouble peeling eggs. Logical if you think about it.
As for the banana thing… I agree; pretty lame.
I think you’ll find the membership on this board skews lots old enough to remember a time before cell phones. I grew up with a party telephone line, and think smartphones are the best invention since hot showers.
Cell phones useless? I challenge you - watch any old pre-cell cop or private detective show. You’ll realize how much easier it would be if the characters could be in constant contact.
Sure, I get that. But either I’m cracking raw eggs wrong or am a hard-boiled shell-removal savant, because there is no way that cracking a half dozen raw eggs and carefully transferring them (without breaking the yoke) to plastic containers is easier than shelling hard-boiled ones.
That said, the banana slicer is way more useless, so I yield. I struggle to imagine what could be easier than slicing a banana with a knife, or how using that device could be faster.
Public phones and landlines were great - as long as you were the user, and the person you wanted to contact happened to be near a landline with a number you know.
But what if you’re meeting your dealer at Penn Station at 3:30, and you need to let him know that you’re running thirty minutes late? Or that an emergency came up and you’re not going to meet him at all?
What if your spouse has left for the store, and you suddenly remember that you need them to pick up a couple extra tubes of lube for the party this weekend?
What if you’re not anywhere near a landline/public phone, and something comes up? Some years ago my motorcycle broke down in rural Wyoming. Not much traffic going by, and there were no payphones or landlines in perhaps a 30-mile radius. I was pleased to find that my cell phone had a signal; instead of hitching a ride to the nearest town, and I was able to immediately call for a tow truck.
IMHO, the cell phone is far from the most useless development/invention.
For most useless development/invention, I’ll nominate GM’s heated windshield wash fluid system. It was so useless that when a problem necessitated a recall, GM didn’t bother devising a fix - they just removed the system altogether and reimbursed owners.
@yendis: And, ironically, alot of the scams and eploitive hoaxes seem to orininate from those same areas.
Since the OP is asking for opinions, let’s move this to IMHO.
Colibri
General Questions Moderator
My vote is for the electric ear cleaner.
If you read the reviews for that banana slicer, you’ll see that it is the foundation for volumes of priceless comedy. 
OP - Do you own a cell phone?
How about a computer?
Why didn’t you just start a thread about how much you hate cell phones, instead of this one?