Just a few links (mostly borrowed from other bloggers) to celebrate the occasion.
The text of MLK's famous "Letter from Birmingham Jail".
Audio versions of some of MLK's most famous speeches:
The speech he gave at the 1963 March on Washington (the "I Have a Dream" speech) (via)
His 1967 speech against the Vietnam war
The speech he delivered the day before his death. (Bonus: Bobby Kennedy's speech about MLK's assassination, delivered (of course) one day later.)
And, finally, a 1956 comic book, Martin Luther King and the Montgomery Story, available on-line. (via)
(Update: new link added.)