In pre-Internet days, I thought up a recumbent tricycle propelled with rowing motions. I never followed up on it. After I got on the web, I found that there are many different designs for rowing bi/tri/quadra-cycles, including at least one over 100 years old. They all have problems, including one common one: it’s very hard to steer with your arms and row at the same time.
Just last month my car was sitting in the naked driveway with a threat of hail approaching. I wondered what I could use to cover the car. At least I wanted to protect the windsheild.
Why isn’t there a rubber like cover that could deflect hail? Thick enough for big hail but light enough that I could manage, get on the car, and store in the trunk.
After doing a search there is a hail cover for about $500.00. I know that’s cheaper than car damage but it doesn’t seem thick enough for big Oklahoma hail. Who knew there was a hail alley? Tornado alley yes, but I didn’t know I lived in hail alley.
I recently came up with an idea of how to deal with spot-treating weeds with pesticides…only to find out that farmers call this “novel” method wiping.
Ditto.
It may be off-topic, but developing and explaining image-processing algorithms I was often asked “But maybe someone’s thought of that already.” My answer was usually “Let’s hope so; if no one’s using the idea it’s probably because it doesn’t work!” 
The California Highway Patrol already has this installed in their cars; it’s called a loudspeaker. I learned this on the 880 Freeway when I heard a loud “Slow down, buddy. I’d pull you over but there’s a guy ahead going even faster than you!” (CHP travels California highways at very high speeds ticketing anyone they overtake only with difficulty. The way to avoid tickets is to travel even faster than their very high speed! … ETA: But that was then. I’m now a very cautious and polite driver. 
)
I came up with the idea of a self contained yoghurt maker for the kitchen about 5+ years ago. Then I seen someone was selling them late night about a year or so ago.
Funny thing is, I didn’t want one.
It happens to me a lot, although I can’t recall any notable examples at the moment, because I just move on to something else.
I do have a couple of ideas in the pipeline at the moment which I think may be actually sufficiently novel (obviously can’t divulge them though).
I made a pizza with using breaded chicken cutlet instead of crust, tasted it and it was great and though I invented a new food dish, but then again like I had this before - oh yeah chicken cutlet parmesan :smack:
When the CD was in its infancy and most cars had cassette players, I thought it would be cool to have a cassette-shaped device that could plug into a portable CD player so I could – drumroll please – listen to CD’s in my car! Wouldn’t that be great?!
I really started to put some thought into how I could make such a thing work, when I saw exactly that device for sale at Radio Shack. Fortunately I suppose, there was a very short window of relevence for this product, so I probably wouldn’t have made millions anyway.
An acquaintance independently invented a fire safety device before discovering it was already patented by someone else.
Garage fires occasionally kill homeowners (and sometimes even firemen) when the heat, which collects near the ceiling, causes the garage door’s main spring to fail catastrophically. This lets the garage door fall closed, and holy shit, if that big spring ain’t helping you lift that door, you’re not gonna lift it up on your own. If you can’t escape into the house, you will shortly die from smoke inhalation and/or burns before anyone can punch through that door.
So the device is a solenoid-driven pin that is normally extended through the garage door’s guide rail and blocks movement of the garage door’s wheels. When you activate the garage door opener, the solenoid is energized and withdraws the pin, permitting garage door movement; as soon as the garage door shuts down, the solenoid de-energizes, and the pin pops back out. If the spring ever fails for any reason, the pin will keep the garage door from falling down. It’s cheap, requires minimal modification to the garage door (some mounting holes, a shear pin thru-hole, and some wiring), and could be a lifesaver. I don’t recall who has the patent, but of course so far it hasn’t been commercialized (or made a code requirement).
Well just because someone else is doing them, doesn’t mean we can’t do it better. Want to go into business?
I meant to point this out to you in the other thread, but forget. It’s for doing exactly what you wanted to do. I saw it on Ask This Old House a few years ago.
On a long drive, I worked out in my head how to build an IC engine without a reciprocating piston. After overcoming all the issues, I had pretty much developed a rotary engine.
Also, after learning about the Stirling cycle in college, I developed a “free” energy source but using the cool temperatures below ground and the heat of the sun as the temperature differential to power a generator. Geothermal Stirling engines are already in use in 3rd world countries where there’s no existing electrical grid.
I’ve got some great ideas - just too late!
All the time. For example, I envisioned the printer/scanner/copier/fax long before they became available.
The solar-powered car cooling fans, my idea before they ever became products. They don’t seem to be succeeding that well though.
The beanbags with camera mounts on them, my idea. Of course they already existed when I went searching for them.
My son just got into lego building at age 5 and I got really tired really fast of him coming up to me repeatedly asking “Dad, can you get these apart?” when the tiny little bricks wouldn’t budge from eachother.
I took out a notepad and started to sketch some drawings of a tool to seperate legos. Mine was a hinged two piece tool that I immediately thought of making a test model and then thought of selling them on e-bay if they worked.
Then it occured to me maybe something already exists and immediately found a much simpler cheaper tool already available. Lego seperator
When I was a kid, my neighbor used to be a part time gadget dude and would come up with wild ideas. Once he showed my father his great idea: A throw away razor (just like what is being sold everywhere now) except his even had shaving cream in the handle.
It was many, many years before one came on the market - but I remember my father saying back then, “that isn’t a bad idea at all…”
As an ex-Computer Science prof. with a long research record, I was basically a paid inventor for a long time. So coming up with something and it turned out it was already done was par for the course.
E.g., early-ish in grad school I figured out a “new” result in Computational Complexity that wasn’t in the books. (And all the CS textbooks at the time were practically brand new.) I went to my advisor and told him about it. He promptly went over to a stack of preprints and such and pulled out of the pile (without even digging thru it!) a paper a buddy of his had written with basically the same result. The thing had just come out 2 years before.
(The buddy is a Big Name in CS and I got to know him later. He even was in town and sat in on my thesis defense. Nice, smart guy. Ah, the old days, etc.)
Anyway, I was embarrassed, etc. But my prof. was actually happy about it. The way it works is you come up with stuff someone did 6 years ago, then 2 years ago, then no one has done. And now you’re on the leading edge. So I actually earned points with that one. I would encourage my own students later on with this principle: don’t worry if it’s been done before, just work on cutting down the gap of how far behind the leading edge you are.
I invented the lightning rod when I was, I dunno, around 8 or so. My mother tells me I was devastated when she broke it to me that someone else got there first.
I similarly came up with the idea of attaching a modem to an NES so you could play games with someone in another location. Not only did that wind up being invented, it was invented by the time I came up with it–it was just only available in Japan.
Some 20 odd years ago I applied to the BBC technology department for sponsorship for my degree. At the second interview I was asked to come up with a novel idea to improve broadcasting. Totally off the cuff, I suggested a way of transmitting codes within teletext to tell vcrs when programmes started and finished. This would avoid issues when shows overran. I went into great detail about what needed to be included in the codes and how the whole thing should work.
This was at least 5 years before programme delivery control (PDC) was launched. Of course the interviewer, a BBC research engineer, would have been well aware of it and was probably even involved in its development. So he thought I’d read some technical article somewhere and I was trying to be a smartarse. No sponsorship for me!