Richard posted an interesting piece on New Jersey's attempts to crack down on the anonymous gossip site JuicyCampus earlier this week. Anonymous speech has served as a centerpiece of American rights, and was critical to the nation's founding. Publius of the Federalist Papers springs most immediately to mind.
Although political theories that focus on limited government would proscribe restrictions on anonymous speech, it does not necessarily follow that a libertarian society should desire such speech. A free society is dependent upon the good consciences of its citizens to enforce (through non-violent means, of course) certain standards and norms that are felt to contribute to the well-being of the community. If someone goes running down the streets yelling profanities at pedestrians, it may well be his right to do so, but those so unfortunate as to live in this misanthrope's community would exact punishment for these anti-social actions by ostracizing the man, and denying him the comforts and company civil society provides.
This non-governmental and non-coercive enforcement mechanism, however, is entirely dependent upon free information about the actions of others. If the misanthrope from the above-stated example caused his ruckus in disguise, or shouted from out of sight so that his peers could not see him, there is no longer a means by which society may respond and thus discourage such actions. When the actions in question are not mere annoyances, as in this example, but accusations and affronts to the character of other individuals, the situation is graver. It is this concern that led to the Sixth Amendment's clause ensuring that an alleged criminal may confront his accuser, and although JuicyCampus is hardly a court of law, the underlying moral principle is the same.
This is not to say that New Jersey's prosecution of anonymous online gossip is meritorious -- far from it. But it highlights the distinction that we as liberty-minded individuals must draw between a proper government and a proper society.