The Simpsons has aired since December 1989 for 555 episodes to date. It has run for more seasons than Gunsmoke, but that show aired 635 episodes (a tally The Simpsons will not break unless it runs into Season 27). Lassie also ran for less seasons, but aired 571 episodes.
But these pale in comparison to some other scripted shows:
Coronation Street has been running in the UK since December 1960 (8,371 episodes).
Sazae-san (an animated series) has been running in Japan since 1969, with over 6,900 episodes.
Neighbours has been running in Australia since March 1985 (6,870 episodes).
Zenigata Heiji (a police show) ran from 1958 to 2006 (with breaks) in Japan for over 1,000 episodes.
The original Dr Who ran from 1963 to 1989 in the UK, and with the new series included produced 800 episodes.