The folks at the Martin Luther King, Jr., Papers Project at Stanford University continuously update and improve this site. Some sections require registration. Many images, and links to images and sound files. It is maintained by the Seattle Times, and is extremely interesting and informative. The images, however, cannot be reproduced without the permission of the Times, or the respective photographers.
Images are of high quality.
This site can be used for educational or personal uses, and will provide great satisfaction. This site is maintained by a 16 year old history student. However, it is very resourceful, and contains plain text reproductions of historical documents. There is a site search engine. There are no images of historical documents.