You learn something new every day.

Garfield by Paws Inc.
Zits by Jerry Scott & Jim Borgman
Frazz by Jef Mallett
On the FastTrack by Bill Holbrook
Over the Hedge by Michael Fry and T Lewis

Wikipedia tells me this: "At the time of its debut, [Zits] had the widest newspaper release of any comic strip in America (the previous record was set by the Muppet Babies comic strip in 1985)." I only vaguely remember a Muppet Babies comic strip... heck, I only vaguely remember the cartoon show, so it comes as somewhat of a surprise that a spin-off comic strip held such a record.

And thanks to Mike P for pointing me to Over the Hedge, which had SPPs both today and yesterday.