Not as famous as some of his other speeches but one of the great ones.  He gave the speech on March 25, 1965 after leading a march of 25,000 from Selma to Montgomery Alabama.  Of his many life lessons, it must also be said that students of rhetoric would do well to spend a very, very long time listening, reading and thinking about speeches by Reverend Dr. Martin Luther King and his other writing.  There is a transcript here.