The horizontally opposed engine, occasionally referred to as a 'boxer,' isn't exactly the most popular engine. You can find 'em in various sizes on new, , the , and not much else. But it's been around as long as the car itself, and Subaru's celebrating 50 years of using this weird little getup.
Unlike more traditional engines, which align their pistons in a V of varying degrees (V-6, V-8) or a straight line (I-4, I-6), the boxer engine places the pistons and crankshaft on a flat, horizontal plane. The pistons moving in and out look like a boxer throwing jabs, which is where the engine gets its name. In brevity parlance, it's referred to with an H (H-4, H-6).
(It should be noted that a small number of horizontally opposed engines lack symmetry and wouldn't be considered boxers. But those are few and far between.)
Benefits of a horizontally opposed engine include good internal balance, short engine lengths, a low center of gravity and more efficient cooling (on air-cooled motors). But it's expensive, and it's also quite wide, which turns simple tasks on other engines -- say, changing the spark plugs -- into a bit of a tangle. Exhaust piping is also more difficult to arrange.
The baby-daddy of the automotive industry, Karl Benz, first built a boxer engine back in 1897. Porsche has been using flat-four engines since before WWII. As for Subaru, it first used its boxer engine in May 1966, attaching it to the Subaru 1000 compact car. Since then, Subaru (along with Porsche) have become the standard-bearers for flat-four and flat-six engines. Break out the cake and the candles!