Anarchism, Marxism, Communism, Leninist Socialism (that type of socialism is far left, but not all others are far left, that's just a rumour made up by pissed off rightwingers or people on the wrong side of history)!
Regular leftism, like Social Democracy and Democratic Socialism, works fine, why have more extreme versions? Edit
The reason why Marxism was founded was because, in the 1840s, Karl Marx was pissed about Capitalism and how shittily the workers in the Industrial Revolution were being treated, so went all extreme and wanted to have a government-less, money-less society where people just happened to help each other out... but of course this would fail, because people need an incentive to do stuff, like getting famous, and that's fine... but in Marxism it isn't fine, so it fails.
The reason why Communism was founded was because Stalin wanted to turn Marx' ideas into an excuse for him to get lots of power, thus Communism is I guess like Marxism but with a brutal despot.
The reason why Lenin made Leninist Socialism is because he wanted to fulfil Marx' prophecy that Capitalism would turn to Socialism and Socialism would turn to Communism.
Here's where the age-old "work ethic" argument kicks in Edit
The Far left fails 100% of the time because it does not give people an incentive to work harder, thus workers lack work ethic and start slacking instead of working hard, knowing they won't get any extra for working hard.
Unfortunately, many US rightists seem to be operating under the delusion that all leftists are far commies or somethings, they think that any trace of a social or socialist program makes people lazy!
This is, however, total bullshit! Many lefties are in favour of raising the minimun wage, thus increasing people's will to work, cuz benefits won't be that high, will they? (Hopefully not, anyway)! Universal Healthcare will reassure people that they won't go bankrupt if they get injured at a job that they are passionate about and/or a job which needs to get done. And free school will assure that the freshmen can get on in life and NOT get burned in debt, thus increasing the economy.