Not part of a main cast, but a character that appears off and on throughout a series. I'll throw out a few.
On The Simpsons:
Comic Book Guy
Jack Gellar (Elliott Gould)
Harry the Hat
Buddy Ryan (John Astin)
Bob & June Wheeler (Brent Spiner & Annie O'Donnell)
Just some to throw out there. Every series has them, as long as the series has a decent run. Who are some of your favorites?