The inline 6, flat 6, and V12 are inherently balanced. Translation: if all the components weight the same, and no crankshaft flexing occurs, all forces cancel for complete balance. In actual applications, crankshafts flex so nominal countweights are added.
The V8 can be completely balanced, requiring crankshaft counterweights.
Inline 4's and V6's engines require additional countershafts running at twice engine speed to balance forces.
The flat 4 engine's primary and secondary forces balance, but there is a resultant couple (torque).
The Bosch Automotive Handbook had a table of engine balance characteristics.
Also see
http://en.wikipedia.org/wiki/Engine_balance
