Every time I watch this speech I get goosebumps.
I often forget that the namesake section, the "I Have a Dream" section – of this speech was ad-libbed. It's kind of cool though, you can see it when you watch it. He reads faithfully from his notes for most of it, and at about the 12:00 mark starts to ad lib and search his head for the right words (at times looking quite terrified that he might go blank!). Then he goes back to his prepared text at about 14:45. And so, we get possibly the most iconic 3 minutes of public speech since the Gettysburg Address – from pure emotion.