No one mentioned that a 2 cycle engine is actually simpler than a B&S engine. Well it is but I still say the B&S is the simplest most common engine. The B&S is a 4-cycle engine just like in most cars. However the 2-cycle single cylinder gasoline engine is even simpler. It has no valves, camshaft, lifters etc and depends on oil in the fuel for lubrication. Found in things like chain saws and leave blowers, they are beginning to be phased out due to pollution issues.
But hell with that, here is a kitty picture from earlier in the day: