I mentioned earlier that "twice in modern history" convention speeches had elevated the speaker into the ranks of future presidential contenders. The two I had in mind were Ronald Reagan's at the 1964 GOP convention in San Francisco and Barack Obama's at the 2004 Democratic convention in Boston.
Readers Frank Gallagher and Scott Rifkin point out that, depending on how you define "modern history," two more speeches might go on the list. They would be Hubert Humphrey's brave pro-civil rights address at the 1948 Democratic convention in Philadelphia, and William Jennings Bryan's renowned "Cross of Gold" speech at the 1896 Democratic convention in Chicago. Bryan technically doesn't qualify, since he was the presidential nominee that same year. Still, it was an important convention speech.
Some video clips of Humphrey delivering that speech here. Amazingly, the Encyclopedia Brittanica has a 35-second audio clip of Bryan giving his speech 112 years ago, including a few seconds' worth of video of him speaking, here.
For what it's worth, three of these four Big Speeches were followed by the defeat of the candidate chosen at that convention. Bryan lost to William McKinley in 1896; Goldwater to LBJ in 1964; and of course Kerry to GW Bush four years ago. Only Harry Truman held on for victory, over Thomas Dewey, after Humphrey's speech in 1948. Which we'll take as an omen if McCain loses this year, and an anomaly if he wins.