CICS

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.

Though definitely not sexy from a hacker's perspective, mainframes running CICS and DB2 are the basis of everyday operations of banks and financial institutions, handling billions of transaction per day. In fact, we can easily say that s/390 machines are built with CICS / DB2 performance in mind.

CICS itself acts as a transaction handler, offering a wealth of resources that are usually not matched even on today's so-called application servers, eg WebSphere.

CICS offers 3270 maps (the mainframe screens you see in banks and flight reservation shops) and services, a session handler and unrivaled transaction and disaster recovery capabilities. Also, when you complie a COBOL program, you can embed DB2 SQL in it via CICS, and it gest compiled to a number of really fast to execute low level calls to DB2.

It may not be sexy, and not so fun to program, but it's definitely a workhorse for real-life apps.