What is called Dr. Martin Luther King's "I Have a Dream" speech was delivered at the Lincoln Memorial on August 28, 1963. Here is a link to a video of it; here is a link to the text. According to Wikipedia, a 1999 poll of scholars of public addresses (talk about a niche field!), voted it the top public address of the 20th century, and it is easy to see why that would be the case. I heard it on TV it as a child when he delivered it and am still deeply moved by it. I have never heard anyone who is remotely close to Dr. King as an orator, while the profound justice of his message and his own deep personal integrity make the speech a seering experience. When I worked in the Copyright Office, the deposit copy of it was on display and it seemed a sacred document. Few, though, will recall today that when he bravely expanded his message to include opposition to the Vietnam War, he was subject to great criticism from many quarters, resulting in great personal suffering and no small degree of marginalization, but that didn't stop him from speaking out.
Parts of the I Have a Dream speech, including the famous ending - the I Have a Dream Refrain and the conclusion - were given in a very substantially similar form in a June 23, 1963 speech in Detroit at the invitation of the charismatic Rev. C.L. Franklin (Aretha's father), available here. That link, eventually to the MLK Papers Project states at the end, after a copyright symbol: "The words of Martin Luther King, Jr. are copyrighted by the King Estate," a claim that is subject to disagreement.
There were in the past claims that parts of King's speech were taken from a 1952 speech to the Republican National Convention by pastor Archibald Carey, but the similarities are extremely slight: both end by reciting the first verse of Samuel Francis Smith's 1832 (and therefore pd) hymn "America," along with the listing of geographic locations from which the orator shouts "Let freedom ring."
A year ago I did a post on the copyright cases involving the speech, available here.