Historically, research tends to find more political intolerance from conservatives than liberals. You can trace this back to the early studies of authoritarianism, which has its roots in the rise of fascism.
A new study, published in Political Psychology, challenges this view.
The authors found "that both political liberalism and conservatism predict intolerance of politically opposing targets" and the reason for this is "perceived threat." Both liberals and conservatives were biased in the expected direction (conservatives intolerance of liberal positions and vice versa). The authors note their findings are consistent with theories of motivated reasoning (a favorite research area of mine).
Basically, both liberals and conservatives are motivated to "afford more rights to groups and individuals they agree with than those they do not."
As a journalism guy I'm more of a radical moderate, meaning while the folks on the far right and far left make for great news stories, I don't think much of their politics. This study really says something we all know, even if the research hadn't supported it, that when you find yourself way out on an ideological limb, you're a helluva lot more likely to be intolerant of the other end of the limb.
A night watching MSNBC and Fox News will convince you of this.