I couldn't find any good images, but Jenny Agutter, Roger Corman, and John Landis appear as Darkman's doctors. Plus Bruce Campbell pops up at the very end as one of Darkman's faces.
Edmond O'Brien in the tag shot of "The Greatest Show On Earth" holds the record for latest appearance in a movie one can make! The film also had Hope/Crosby cameos too.
Manderley reminds me that when Dean and Jerry remade Bob Hope's GHOST BREAKERS as SCARED STIFF, Bob and Bing made a gag appearance at the end.
Bing and Bob of course appeared countless times as gags in each other's movies. My favorite is probably MY FAVORITE BRUNETTE where Harry the executioner (Bing) is upset when told that they've exonerated Bob.
Whilst strictly not cameo roles, James Cameron has "appeared" in on way or another in several of his movies;
Terminator: Cameron did the voice of Stan Morsky, the Porsche owner who cancels his date with Sarah Connor.
Aliens: Genuinely, Cameron provided the "voice" of the alien queen.
T2: Having obviously got a taste for contributing odd voices to his characters, Cameron provided the cries of the T-1000 as it died in the molten metal.
True Lies: Cameron provided the voice-over of the Harrier Jet pilot asking "these missles aren't going to set off the nukes are they?"
Avatar: And another 'copter pilot voice-over, as Cameron was one of the voices of a pilot for SecOps before they attacked Hometree.
Titanic: And actual appearance this time, albeit only his hands. When Jack draws Rose, that's actually Cameron doing the sketch.
