Here’s a topic to share new, unconventional, innovative human-powered vehicles, whether there are prototypes or mass-produced.
The velofeet, a unicycle/wheelchair hybrid aimed at low-mobility people. Invented by Manuel Álvarez-Icaza Icaza.
A few pictures:
The inventor admits that it’s not convenient to go up a slope.
Found on Inventor creates Velofeet unicycle to tackle city congestion | Daily Mail Online
The orbit wheels, invented by Shane Chen (who also invented other things, including the unicycle, which has been talked about here and here).
A bit like a caster board, but where the two feet are completely independents.
It kinda looks like freeline skates:
The halfbike, created by a company named Kolelinia. Two versions were released.
This video made me want to try, but I didn’t have the chance! It’s expensive, even used ones…
As more people tried the Halfbike I realized that the engineers really had created something totally new. This felt nothing like a bike, or a scooter, or anything. Imagine if you had never seen or tried a bicycle before, how skeptical you would be about staying up on it. Just like a bike or a unicycle, the Halfbike becomes second nature once your brain figures out how it works.
Version 2:
The Heelys, created by Roger Adams in 2000, which are basically shoes with a wheel under the heel.
Some guy using heelys and scooter:
I’ve used a halfbike, and as a unicyclist I don’t see the point of it. It’s not great for hills, bikes are way more efficient, unicycles are more fun (imo)…
In addition to unicycling, I sometimes commute via a penny farthing
or other times a kick scooter with over sized wheels with pneumatic tires (a.k.a. a footbike).
No fun then? What a pity. 
The Gauswheel. I didn’t know that it has been succesfull enough to make the company improve them… They now build electric ones. Website:
https://www.gauswheel.com/
I own a Gauswheel, but a much earlier version with a 26" front wheel and the small secondary wheel on the back instead of the front.
Based on how it rides and how useless it is I’m also surprised that the company still exists! It’s kind of fun as a BC training device though (you can easily lift the back wheel off the ground).
I also own freelines (OG aluminium ones) which are pretty fun and moderately practical if you have smooth flat surfaces to ride/commute on. I don’t think the Freeline company exists any more, with previous employees moving to JMK. It’s the place to go if you want some new ones.
Less weird than some of the others but there’s the Bicymple that seems like a slightly more serious circus bike:
So that’s essentially a unicycle with the training wheel in front? 
Now only 799 USD
Doesn’t sound too bad for a 29" wheel with a freewheel hub and disc brake (and some random bike-like parts for free)
Wow, the now offer even a penny farthing version of it with Schlumpf hub for only $1,499.99 … so the bike is basically almost for free. Unfortunately it’s out of stock.
and …
Proprietary Schlumpf hub, 2-Speed, Freewheeling, Black
they offer a freewheel Schlumpf! How cool is that?
1500 USD for a freewheel schlumpf, laced into a 36" wheel.
Perhaps you can even directly mount a seat(post) to the front fork and you’re good to go!
how do you stop? nevermind…
On the Gauswheel, or the bicymple?
On the Gauswheel, some of them have rim brakes, but personally I’d never risk taking it on a hill!
I suspect that if you got really good you could maybe do a sideways skid stop grinding away some of the footplate plastic though…
Olympic monowheel
like a unicycle but the rider is in the center of the wheel
That is one of the most disturbing things I’ve ever seen…
I have seen one of those before. Years ago. It now occurs to me that this and the half-bike are basically a mirror of one another at the base and with the seat and handlebars exchanged on the top half
