I found this history from Ken at Popehat to be incredibly useful background on free speech jurisprudence from OW Holmes, particularly of the oft-abused "yelling fire in a crowded movie house."
Perhaps the worst modern threat to free speech is this notion that "hate speech" is somehow a class of speech that should be banned. Well, I would ask advocates of this position to remember that nearly every bit of political speech is hate speech to someone. Those who advocate for such a restriction generally imagine themselves defining what is hateful. Which leads me back to my #1 legal test: if one supports some sort of government-rule-making legislation, imagine the politician you like the least making the rules. If that makes you queasy, you shouldn't be supporting that legislation.