Another is the EcoMotors International (EM) opoc engine, aka open piston, opposed cylinder. Here's how it works. Instead of an I or a V pattern, EM's opoc is laid out like a two-cylinder boxer engine. However, each cylinder contains two pistons, and they are facing each other. This gives you four rods turning the crankshaft, with no cylinder heads and no valve train (for the record, we're not sure how fuel/air enters and exits). The opoc engine is also a two-stroke, guaranteeing lots of quick torque, but it "runs as a fully balanced 4-cylinder 4-stroke engine."
Shrinky-dinkying the engine has a whole host of other benefits that EM lumps together under the banner, "power density." They include: lower weight and smaller size, fewer materials, less friction, higher MPG, lower missions and less heat rejection. And as the engine is working against itself, there's plenty of built in noise cancellation. Power? Burning diesel, two 100 mm cylinders produce 325 hp and 664 lb-ft of torque @ 2,100 rpm. Nice. And thanks to Eugene for the tip!