Tom Hanks and Green Day play themselves in the Simpsons movie but it probably wasn't too memorable because I had to look it up.
In Sunset Blvd Buster Keaton, H. B. Warner, and Anna Q. Nilsson
played themselves as "the wax dummies" and very famous director Cecil B DeMille played himself. It was uncomfortable because the directors didn't really like each other lots. He did a good job.