One of the slides by some IBM consultants presenting at the financial firm I used to work stated "Western Civilization runs on the mainframe." Considering how much banks still use mainframes, this is probably true.
From what I know, considering it's not "just" banks but pretty much any long running financial business (IE insurance companies), I believe it's one of the most used but least talked about technologies out there.
It was pretty fascinating to see how it all came together. Also despite the languages age after analyzing it for a length of time it was clearly elegant for that type of financial processing.