The
Customer Information Control System, created by
IBM.
It is a strange mixture between an operating system and an application, actually called a transaction processing system.
It is really old.
You can program it in COBOL, PL/1 or mainframe assembler.
Bet your bank uses it, and because it works great they never cared to replace it, causing the millennium problem.
Although I hate it, it works fine and you earn a lot of money programming it.