I think most Americans are unaware of the existing limitations on freedom of speech. I used to think that freedom of speech was the most basic and absolute Constitutional freedom; that there were no limits on it, period. Boy was I wrong. Here are some miscellaneous discussions/points about what speech isn't constitutionally protected—I wrote most of this for a mailing list a few months back, but I think it fits the current topic pretty well.
The decision in a 1999 Free Speech Coalition case (PDF) (URL updated in 2008) says:
The First Amendment, however, does not protect certain limited categories of speech that are "utterly without redeeming social importance." . . . See also R.A.V. v. City of St. Paul, 505 U.S. 377, 382-83 (1992) (stating that "[f]rom 1791 to present . . . our society, like other free but civilized societies, has permitted restrictions upon the content of speech in a few limited areas, which are of such slight value as a step to truth that any benefit that may be derived from them is clearly outweighed by the social interest in order and morality"). These categories include obscenity, Roth, 354 U.S. at 483, libel, Beauharnais v. Illinois, 343 U.S. 250, 266 (1952), and "fighting words," Chaplinsky v. New Hampshire, 315 U.S. 568, 571-73 (1942) [see below. —jed]. Child pornography is also one of these categories of speech. Ferber, 458 U.S. at 763-64.
This article about free speech on the Internet says:
The area of speech defined as clear and present danger was codified in the Supreme Court case Brandenburg v. Ohio [actually, it was codified long before that, but see below for more info. —jed], in which the court ruled that a law may only forbid speech advocating illegal or violent actions if the speech is intended to incite or produce imminent lawless action and if the speech is likely to incite or produce such action. (Cavazos and Morin, p. 74) Fighting words are a continuation of the clear and present danger idea, and are defined as words which are so offensive and abusive that they are likely to "cause or incite immediate physical retaliation by the audience." (Cavazos and Morin, p. 76)
The usual example of something that presents a "clear and present danger" is falsely shouting "fire!" in a crowded theatre (even though that doesn't really fit the description given above)—see below.
In Chaplinsky vs State of New Hampshire (1942), the Supreme Court ruled that New Hampshire's law prohibiting certain kinds of "fighting words" speech was constitutional. (It's a surprising opinion to me. It closes with statements like "Argument is unnecessary to demonstrate that the appellations 'damn racketeer' and 'damn Fascist' are epithets likely to provoke the average person to retaliation, and thereby cause a breach of the peace.")
The "fighting words" exception was later used to try to stop the American Nazi Party from having a rally in Skokie, IL; the idea was that the Nazis' words were likely to be so provoking that they would incite the audience to violence, and therefore the gov't would be justified in preventing the Nazis from speaking. However, that attempt failed; the court ruled, iIrc (which I may not), that "fighting words" can only be construed very narrowly, as a one-on-one sort of thing, not for a speaker addressing a crowd. Note, too, that most free-speech exceptions (at least during peacetime) don't allow governments to prevent as-yet-unspoken speech that they think is going to be inflammatory; they only allow governments to punish the speakers after the unprotected speech is made. That is, one may be able to successfully argue that a given speech wasn't constitutionally protected, but not that an upcoming speech should be prevented because it won't be constitutionally protected.
The decision in Brandenburg v. Ohio (1969) is generally interesting throughout, but the totally fascinating bit is the concurring opinion of Mr. Justice Douglas (you'll have to scroll up a line or two to catch the beginning); he says that the "clear and present danger" test is applied much too broadly, particularly in peacetime. More specifically, he says: "Those . . . World War I cases . . . put the gloss of 'clear and present danger' on the First Amendment. Whether the war power—the greatest leveler of them all—is adequate to sustain that doctrine is debatable. [395 U.S. 444, 452] The dissents in Abrams, Schaefer, and Pierce show how easily 'clear and present danger' is manipulated to crush what Brandeis called '[t]he fundamental right of free men to strive for better conditions through new legislation and new institutions' by argument and discourse (Pierce v. United States, supra, at 273) even in time of war."
(That test was created during WWI, in Schenck v. United States (1919). Here's more context for the quote I gave last entry: "We admit that in many places and in ordinary times the defendants in saying all that was said in the circular would have been within their constitutional rights. But the character of every act depends upon the circumstances in which it is done. . . . The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic. It does not even protect a man from an injunction against uttering words that may have all the effect of force. . . . The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent. It is a question of proximity and degree. When a nation is at war many things that might be said in time of peace are such a hindrance to its effort that their utterance will not be endured so long as men fight and that no Court could regard them as protected by any constitutional right.")
For a more recent argument against the test, see "Rethinking the Clear and Present Danger Test." I just glanced at the beginning of it, but it looked interesting.
No discussion of free speech in the US would be complete without reference to a fascinating book called Defending My Enemy, by Aryeh Neier. (Sadly long out of print.) [I had originally mistakenly written "Defending the Enemy"; corrected the mistake in 2006 for accuracy.] It's about the American Nazi rally in Skokie, which at the time was a smallish community heavily populated by concentration camp survivors. Neier was the head of the ACLU during that period. (When he was 3, he was spirited out of Nazi Germany; his parents died in the camps.) He presents a series of carefully crafted, serious arguments against allowing the Nazis to speak—and then thoroughly demolishes each such argument. Really good book, very affirming of faith in the First Amendment. I urge anyone interested in the topic to go to your local library and read the book—order it by inter-library loan if you have to. I don't agree 100% with every single things Neier says, but I agree with at least 90% of it, and it gave me a much better understanding of how free speech really works in the US, and what is and isn't legal.
Btw, in that book Neier comments that during the period of the Skokie incident, a lot of liberals left the ACLU—they felt that defending the Holocaust survivors in Skokie from having to hear the Nazis was more important than defending the Nazis' freedom of speech. This was the most surprising part of the book for me; I've always been a rabid liberal, and always assumed that defending the First Amendment was part of the liberal agenda. (If only they'd let me see the agenda when I go to those Liberal meetings!) This may mark me as naive, but it hadn't quite occurred to me that mainstream liberalism and the ACLU might find each other at odds. . . .
Anyway, as my pal Jack Mantis is fond of saying, "Most people are in favor of freedom of speech—for themselves and other correct thinkers."
The best answer I've seen to what to do about speech that sparks dangerous ideas is to counter it with more speech, speech that reduces the danger of those dangerous ideas. The Jesuits, as I understand it, teach all the heresies, along with counterarguments to them. This approach makes way more sense to me than simply banning the heresies—it results in more knowledge and more understanding, rather than more fear and more ignorance. I suspect that I would have strong doctrinal differences with the Jesuits, but I have a great deal of respect for their intellectual tradition.
I suppose that to be fair, I need to give equal time to the Devil's Advocate, who notes that these exceptions to freedom of speech have been around in the US for over 80 years and haven't yet resulted in a fascist dictatorship; a little more limitation on freedom of speech probably won't destroy our democracy. Moreover, countries that do not have our free-speech and free-press traditions (such as the UK and Canada) seem to do pretty well.
Nonetheless, I think that this is one area where slopes can be very very slippery. I've always been fond of Benjamin Franklin's line that's usually paraphrased as "Those who would trade a little freedom for a little security deserve neither." (The original line, less pithy, was "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.") I'd rather err on the side of a little too much freedom of speech than on the side of too little. I know that there are restrictions on what we can say and write—but I don't feel that that fact justifies imposing more such restrictions.