According to a disturbingly pleasant graphic from Information is Beautiful entitled simply 20th Century Death, communism was the leading ideological cause of death between 1900 and 2000. The 94 million that perished in China, the Soviet Union, North Korea, Afghanistan, and Eastern Europe easily (and tragically) trump the 28 million that died under fascist regimes during the same period.
During the century measured, more people died as a result of communism than from homicide (58 million) and genocide (30 million) put together. The combined death tolls of WWI (37 million) and WWII (66 million) exceed communism’s total by only 9 million.
The graph can be located here. The author of the Reason article also notes that several of the largest famines occurred under communist regimes: 9 million in the Soviet Union (1921-22), 30 million in China (1958-62), and 3 million in North Korea (1995-99). Since we know that the famine in the Soviet Union was a result of the communists consolidating power, that the Chinese famine was part of Mao's "great leap forward" and North Korea because of communist central planning, that brings the total deaths due to communism to at least 136 million. Of course, this doesn't include war deaths caused by communism (the Chinese Civil war of 1945-49 left 25 million dead; the Korean War had 1 million; the Vietnam War 2 million; the Soviet-Afghanistan war killed 1 million; the Russian Civil war of 1917-1922 killed 9 million), or the 2 million killed by Khmer-Rouge in Cambodia. So, in round numbers, some 176 million died as a direct result of communist ideology or expansion.
Of course, it is unfair to leave out other socialists. According to the chart, Fascism was responsible for 28 million other deaths in the 20th Century, not including wars. WWII resulted in 66 million dead, the vast majority being the result of the conflict between Germany and the Soviet Union.
So, all told, 270 million deaths directly attributable to socialism. This, of course, doesn't include the millions of abortions or deaths due to environmentalists (such as the malarial deaths because of the banning of DDT).
Another way to look at this is through the perspective of religion. Communism (in fact, socialism generally) is an atheistic movement. So, you could say that Atheism accounted for some 270 million deaths in the 20th Century.