Frasier and Cheers star Kelsey Grammer is such a frequent visitor to Springfield that it seems ridiculous to refer to him as a mere 'guest' of the show. Grammer has given his distinctive voice to murderous villain Sideshow Bob a grand total of 12 times since he first appeared on The Simpsons way back in 1990.
Grammer isn't the only guest star to play recurring characters in the show. Godfather Part III star Joe Mantegna regularly voices mobster Fat Tony and, more recently, Malcolm in the Middle actress Jane Kaczmarek has played the bitter Judge Constance Harm on several occasions. The late Phil Hartman (Lionel Hutz, Troy McClure) and Marcia Wallace (Mrs Krabappel) have always been credited as guest stars, while funnymen Jon Lovitz and Albert Brooks have also voiced several characters in the show's long run (the latter most memorably voicing Bond villain parody Hank Scorpio in season eight).