If we're going with Silver Age and Golden Age here, let me see what I can think of:
SIMON AND KIRBY: Appeared in one Boy Commandos story as themselves. The one with crossovers from every other strip they drew (Sandman, Guardian, Newsboy Legion, etc.).
JERRY SIEGEL: Cameoed in one issue of BATMAN in the Forties, though he didn't write or draw it.
BILL FINGER: As Black Hand, the villain of the Silver Age Green Lantern.
GARDNER FOX: In the Detective Comics story, "The Strange Death of Batman".
JOHN BROOME: In the Detective Comics story, "The Secret War of the Phantom General."
BILL GAINES, AL FELDSTEIN, and MOST OF THE EC GANG: All over the place in their mags, and later, often in MAD.
DAN DE CARLO: Along with Stan Lee, in at least one Kathy story in the Fifties.
JACK MILLER, E. NELSON BRIDWELL, MORT WEISINGER, MURRAY BOLTINOFF, MIKE SEKOWSKY, GIL KANE, MURPHY ANDERSON, and PROBABLY A LOT MORE: In the last ish of INFERIOR FIVE written by E. Nelson Bridwell.
DENNY O'NEIL and NEAL ADAMS: Cameos in "Night of the Reaper" (at the party) in BATMAN.
STEVE DITKO: In "How Stan and Steve Create Spider-Man" in AMAZING SPIDER-MAN ANNUAL #1.
BOB HANEY AND JIM APARO: In that BRAVE AND BOLD Batman / Sgt. Rock teamup that's been discussed.
That's about all I can remember right now.