My personal favorite out of all the carts I've found is the packwheel:
http://www.packwheel.com/
I have penciled out multiple copycat builds over the years, but most involved a level of fabrication I was not willing to fund or mooch off of friends. I'm a firm believer that if you just mull it over long enough a partially complete version can be found and adapted with much less effort then a ground-up build many times.
With this project it didn't dawn on me until I recently upgraded my bike to a Surly Ogre. This bike has fork mounts for mounting bottle cages or cargo cages for bikepacking or touring. While brainstorming on just how much gear I could conceivably pack on a mountain bike front fork all the pieces fell into place.

(not my bike or picture, but illustrates fork cargo cage)
Within a few weeks I finally stopped by my local sketchy bike shop (the kind with piles of rusty bikes and employees with prison tats) and bought a pile of parts. Interestingly enough the guy at the shop caught on faster then I would have imagined, he mentioned using "hippy wheel barrows" to move large camps, which sounded a lot like the Game Tote cart from the resource thread.

Parts List:
front fork
rear cargo rack
26" downhill wheel
tire
stem
rear swept handle bars
brake lever/cable/housing
brakes
total cost was $40 out the door. doitnstyle1 hooked me up with a tube and some more brake parts and I was in business.

assembly was pretty straight forward, I flipped the fork upside down so the (I assume) fender mounts where facing up for the rear rack. Drilled through the steer tube to mount the rear end of the rack.

and just like that I'm off and running. Rack means easy pannier interface if that is your scene, drybags, milk crates or buckets could be lashed on as needed as well.

Load stays centered over the wheel, steering and navigating stairs and obstacles is a snap since there isn't the tipiness from dual wheels.
First real test run was the .75 mile walk to the gas station to restock my backup gas:

While I'd originally schemed on bolting jerry can carriers somehow I found just a single strap over the rack did the job just fine. Loading and filling them this way is annoying, but not hard. I was fully expecting to have to fill the cans a few gallons at a time going back and forth to keep the cart balanced (I don't have a kickstand for it, had to just hold it while they filled) but I actually got to about 4 gallons before I even noticed the cart pulling to one side much. It would seem balancing the load might be much less of an issue then I'd imagined.

trying to set the cart down to go in for a soda wasn't ideal, not sure a kickstand would even work that well with 80lbs, but if the load can pivot a bit like these cans they can make contact before the cart is fully laid down.
walking back proved a breeze, on flat paved ground you really can push 80lbs with two fingers. The way I had the load mounted it had some room to shift back and forth a bit and you could tell the difference, but there was a pretty good margin for error without really messing up the balance of the cart. On level ground the brakes aren't needed, stopping 80lbs was no work at all but of course hill testing is in the future. If I can haul 50lbs without really needing brakes I might just leave them off future versions.
picking the proper bar and stem for this project required the most thought, and is about the only thing keeping any $20 bike at the thrift store from being a good candidate for a cart build. I really do think the swept back bars are the best option, and I wouldn't mind if these bars where longer. I found I had to adjust my stride a bit to keep from kicking the back of the tire, a longer stem could also fix the issue.
Total weight of the cart is about 10.5 lbs, this and its relatively compact form give it a big advantage over some of the larger, bulkier carts out there. Popping the wheel off would make it very compact for keeping in a car for contingency transportation, and its light weight would make crossing terrain it couldn't navigate easier (stream crossings for instance).
Conceivably a cart/packraft combination would open up a lot of terrain that would otherwise be untenable, even a 5lb raft is too great a burden for most backpacks unless the trip is planned around rafting, but with a cart a 5lb contingency option is much more easily accommodated. "Bikerafting" trips are increasing in popularity, but I feel a cart would have access to some more terrain then a bike would, filling a gap between bike and foot travel.

This is of course hypothetical spitballing on my part, but a packraft might finally be within the budget and maybe I can find out for myself someday:
http://www.diypackraft.com/shop/
Being made from 100% bike parts makes finding replacement parts a non-issue, and just building a new one from scratch requires little more then some allen wrenches so this is basically a disposable piece of gear, you're not out much if you have to ditch it somewhere.
I selected my parts based on price and availability alone, with some careful shopping a much better version could be made. By using a 29er fork with the right spacing you would have the option to fit nearly any front bike tire from BMX to beach cruisers. Before I started I had looked at getting a fat bike fork so I would have the option for wide tires, current prices put the fork at several times the total cost of my build but as they become more common I'm going to be keeping my eyes out for a cheaper fork and wheel so I can build a cart with more flotation, traction and some artificial suspension:

Sourcing an exotic ti or carbon fork is going to offer some weight savings, but for most that is going to be absurdly far down the cost/benefit ratio unless you have a good hookup on bike parts. I do wonder what a carbon fork/bar/rim combination would get the weight down too, possibly <5lb.
Obviously this style of cart gives up a lot of load hauling options compared to a flat bed, two+ wheeled cart, but certainly anything you could carry easily in or on a pack could also be carried on one of these. Looking into some of the cargo racks from utility bikes might also give some easier loading options for bulky items as well. The main advantage is maneuverability, I think a one wheeled cart can go many places a two wheeler can't. There are some videos on the Pack Wheel site showing what navigating rougher terrain with a cart looks like, not a walk in the park but doable.
I plan to experiment with the cart as a range extender, a 20lb backpack with essentials and a few contingency items and a cart with 50lbs of consumables and a few comfort items has the potential to offer a tremendous range on foot as long as the route isn't too arduous, something like a 3-season pulk sled.