I make it a point to visit the MLK Memorial every time I'm in Atlanta. It's a national historic site and it's really amazing, yet humbling and inspirational at the same time. Entrance is free (but please make a donation) and the exhibits change from time to time. All are educational, moving, and do a really good job of conveying not only the man behind the legend but a sense of the gross injustices that segregation was. As much as you may know about the time period and Martin Luther King, Jr., you will walk away from the memorial with more knowledge.
A bookstore/gift shop is on site selling memorabilia to commemorate the man and the museum. He is entombed near a large reflecting pool on-site. The home in which he was born is a short walk from the visitors' center, as is Ebenezer Baptist Church, where he and his father preached. Both are certainly worth checking out.
It is impossible to leave the memorial without feeling moved and wondering what he may have accomplished had he been given a few more days, months, or years.