Arthur? It was done by Cinar - now Cookie Jar - which is a Canadian company, and it's got thirteen seasons, having started in 1996. Don't know if that counts though, since it's kind of both an American and Canadian show.
Then maybe Caillou? That show's been around since 1998, although due to huge production gaps, it's only going into its fifth season, so I don't know if that counts either.
Beyond those two I don't know.