On this Martin Luther King, Jr., Day, take some time to check out Stanford University's King Research and Education Institute.
There you'll find a wide collection of Dr. King's works, including this page that has text and audio links to some of his more famous speeches. Naturally, the "I have a dream" sermon, and yes, that's what it was, from the 1963 March on Washington is there.
Too often we consider this speech as one directed only toward African Americans. Take a few minutes to read the whole thing (it's not that long) or better, listen to Dr. King deliver it. It's abundantly clear that it applies to everyone and anyone seeking justice, social and economic. That's why so many people spent today volunteering with service projects to help their communities and neighbors.
The betterment of every single person is a goal our country should never lose sight of and one worth working toward not just this one day a year, but every single day.