Look at, for example, Jack Henry & Associates http://en.wikipedia.org/wiki/Jack_Henry_%26_Associates

Their core banking stuff, which powers the $600 million capitalization local bank I use, is System i (AS/400) based as I understand it.

Right, that's not actually surprising to me, I know banks still use mainframes, and I thought it was well-known. Same about airlines, and heavy-duty control systems (say, sewage control for a large city or something) that are likely to be legacy. I was wondering what could be organizations I wouldn't even suspect.

I believe O'Reilly auto parts has an AS/400 at each of it's stores.