Why do people yell "Freebird!" at concerts?
"Freebird" was recorded by Lynyrd Skynyrd in 1973 for their debut album. (It's also spelled "Free Bird.") It's second only to "Sweet Home Alabama" among the group's greatest hits, and it's debatably second only to "Stairway to Heaven" among classic rock anthems. The album version is about eleven minutes long, and whether or not you know it by name, it's a safe bet that you've heard it.
The practice of demanding Freebird at the most unlikely occasions dates back at least to the 1980s. While it's hard to be certain of the origins—especially if, as we assume, beer was involved—there are two events worth noting.
The first is the release of Lynyrd Skynyrd's live album, One More From the Road, in 1976. The band customarily played "Freebird," but left it out of the concert at which this album was recorded. When the band returned for its second encore, lead singer Ronnie Van Zant asked the audience "what song is it you want to hear?" And the crowd yelled back "Freebird!"
In that instance, they were being serious. On the other hand, in the 1980s, Chicago DJ Kevin Matthews told his listeners to shout "Freebird!" at a concert by former Brady Bunch star Florence Henderson "to break up the monotony." This worked so well that he began looking for other "really lame" shows at which his listeners were told to yell the request. It is uncertain whether he actually started the practice; more likely, he popularized and expanded it.
You may be interested in Rock's Oldest Joke: Yelling 'Freebird!' In a Crowded Theater, an excellent article on the subject from the Wall Street Journal, and this list of 500 Songs That Shaped Rock.