I've eaten at Denny's. It's a pretty solid 24-hour joint that dots the landscape on both sides of the border. It's hard to beat their lumberjack slam. Denny's is a great place to pick up some free WiFi while you munch on some bacon, eggs, and hash browns.

Then, there is IHOP. I ate there tonight for the first time. Don't believe the hype. It's Denny's for pancake freaks. Actually, pancake Freaks. Upper case.
Case in point:

The undisputed champion of 24-hour breakfasts is Waffle House. It's not everywhere in the United States -- you won't find it in New England, most of Illinois, or the Midwest -- but Waffle House owns the American south's Interstate corridor.
Think Elgin Street Diner, but at
every offramp you can find. Also, while Denny's and IHOP are all colourful with their logos, Waffle House is simple.

This one is in Birmingham, Alabama. I've only eaten at the Nashville location (er, one of the Nashville locations. There are several interchanges in the area).