As I said in the McDonald's thread, we don't have quite the same roadside tradition here in Canada. Sure we have fruit stands and Chip wagons and Diners, but not to the extent and diversity of the States. It's one of your cultural highlights. Seriously! Everytime I visit the States, I try to find some local comfort food.
Alton Brown's series
Feasting On Asphault makes for great viewing. I totally want to hit the Maid-Rite in Quincy, Illinois.
The show's Wikipedia entry has a list of all the establishments he has featured:
http://en.wikipedia.org/wiki/Feasting_on_Asphalt
As for around here, Weber's Burgers is the de rigeur stop for those heading up north to Cottage country. They even installed an elevated crosswalk across the 6 lane highway to allow southbound travellers to park and come on over. Their burgers are pretty good, if not particularly special. The appeal is mostly their history and atmosphere.
Trivia Note: The crosswalk used to be at the base of Toronto's CN tower and was sold to Weber's when replaced.