Here's a fascinating webpage which lists the number of deaths in the wars, genocides, democides, and general massacres during the 20th century. Comparing these numbers, which are compiled from various sources (each source named for each deaths figure; I would seriously discount Rummel's figures, which seem to be compiled according to the Marc Herold method; Rummel's book, Democide, is well worth a read, but his numbers to my mind appear wildly exaggerated), to those of the recent Afghan War or to the current Saddam War, should prove very enlightening. Unless your name is Zap or Gas.
UPDATE: This guy, whose name is Matthew White, has a hell of a website called the Historical Atlas of the 20th Century which I just spent a couple of hours clicking through. Give it a look; it gets four and a half coveted Iberian Notes stars.