I remember going through BWI's Concourse D and having a really eerie feeling. You could definitely tell that the place had been a big time hub for Piedmont and USAir back in the 80s. By the time I traveled through it was a fairly empty shell (just a few US, F9, and FL departures).

There's also RDU's Terminal C (which is showing signs of life again as an AA Focus City), SJC's Terminal A (similar story, former AA hub), STL Concourse D(AA again).

I shudder to think about what PIT will look like after US Airways completes it's screwing over of that city (40-50 empty gates, what a waste).

Rumordly DL's Terminal E at DFW will undergo a similar process.

ATL's Concourse C still reminds one of Eastern (although it's now FL's main hub). DEN's concourse A was designed to be CO's Western Hub (now Frontier's hub).