I believe it's gone now, but up until recently Continental Express ran RJ flights from one end of Houston to the other (EFD to IAH and back). This was one of the few scheduled air services in the world that connected co-terminals.

Before 9/11/01, United Express flew IAD-BWI as well, which makes sense because of the often horrendous traffic between the two airports.

I was very surprised when I first heard about both routes, thinking ground transportation would have been the only option for such a short distance!