Fuji Comp 29r, Downtube, Tommi Sea fat bike, Nashbar SS 29r half-fat, Full suspension mini-velo/MTB, Loopwheel M-Velo
Not a folding bike, but this seems to be the spot for 20"ers...
I've had a thing for the Cannondale Hooligan, ever since I saw it on their website years ago. Reluctant to buy anything sight unseen tho', and I've never found a Cannondale shop that had one in stock, so I've held off. In the meantime, I've been intrigued by several forum threads on mini-velos. One particular thread had a bunch of pics of full-size MTB's with 20's that I really liked, so that got the seed planted. Here's mine, just finished this week by my LBS. Local shop had a used Raleigh RAM 1.0 for $150. 10yrs old, but in really good shape, and if things didn't work out well I wouldn't be out too much money. Went with Sun Rhino Lyte 406 x 29 wheels & 1.95 hookworms, on 36 spoke disc hubs. I very happy with it! I was shocked how fast it accelerates, and rides pretty nicely with the full suspension. The 175mm cranks are way too long; I scrape pedals just barely leaning the bike. I also wanted to go with a single front sprocket to simplify things, but on the current old-style setup all 3 are riveted together. So my LBS is ordering a single sprocket & 152mm crank assembly from one of their child-sized mountain bikes.
I call it my supermoto; in the motorcycling community, a supermoto is a dirt-bike with smaller diameter sportbike wheels & tires.