Carrie Fisher: The Big Bang Theory
The great Princess Leia appeared in The Big Bang Theory in an episode where Sheldon and James Earl Jones rang her doorbell and then ran off. She came out with a baseball bat, not realizing they had already gone. Even though her appearance was short, it most definitely is a memorable one.
Adele: Ugly Betty
Another brief appearance was by Adele in Ugly Betty in 2009. Showing off her easygoing personality, Adele certainly fit into the scene perfectly.