Hell man, just about any engine with a hemisperichal combustion chamber can be called a hemi (a lot of mopar guys would dispute this. however)
My old ford escort had a hemi in it; 1.9L CVH single cam 4 (CVH = Canted valve w/hemispherichal combustion chambers) Benifits of a hemi, efficient fuel consumption combined with good power and torque. also, the valves will flow better when open.
the hemispherical combustion chambers on the cylinder head act sort of like the shaped charges around the core of an old atomic bomb, they direct the force of the combustion towards the center of the piston, rather than across a flat surface. Also, on most HEMI motors, the spark plugs go in at an angle, rather than straight in from the top. When Mopar made the first HEMI's they created something unique, and since then, many engines have adopted a similar design for the combustion chambers.