Stars like Will Ferrell, Matt Damon, Oprah Winfrey and Kermit the Frog all stopped by
Played: Dr. Leo Spaceman in dozens of episodes
We're right on the threshold of "recurring" rather than "guest star" when it comes to Dr. Leo Spaceman, but Tina Fey gave her old "SNL" buddy Parnell one of his finest roles as the "30 Rock" equivalent of Dr. Nick Riviera, a dangerously unqualified -- his degree is from Ho Chi Minh City School of Medicine -- physician who somehow ends up treating the whole team at various points. Pronounced "Spuh-chem-in" by everybody other than Tracy, the good doctor's occasionally retrograde understanding of medicine never prevented him from helping Liz, Jack or former clients like Alf, the Unabomber and Kenny Rogers. Dr. Spaceman got one of the show's best sendoffs, when he announced "That's a series wrap on Leo Spaceman, suckers!" after somehow rising to the rank of Surgeon General.
- Daniel Fienberg