Not part of a main cast, but a character that appears off and on throughout a series. I'll throw out a few.
On Seinfeld:
Newman
J. Peterman
David Puddy
Jackie Chiles
On The Simpsons:
Apu
Sideshow Bob
Disco Stu
Ralph Wiggum
Groundskeeper Willie
Comic Book Guy
On Friends:
Jack Gellar (Elliott Gould)
Janice
Gunther
On Cheers:
Lillith
Harry the Hat
Nick Tortelli
On Nightcourt:
Mel Torme
Buddy Ryan (John Astin)
Yakov Korolenko
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?