We all know "I have a Dream." American history books tend to leave out MLK's other accomplishments and causes. We tend to learn in school that MLK fought for civil rights, and then later, he was assassinated. The truth is that MLK was also an activist for labor rights and economic justice, and he also spoke out forcefully against the War in Vietnam.
Here's audio of a speech he gave against the Vietnam War:
And here he is arguing against both communism and capitalism - arguing for a synthesis of the two: