IBM‘s GUI OS. It was popular in the early 1990s when it offered a strong alternative to Microsoft‘s Windows product line, but has ultimately lost its marketshare due to virtually non-existent marketing.
OS/2 maintains a small userbase due to its stability and compatibility with Windows 3.1 – a legacy operating system, no longer supported by Microsoft. Most of that “userbase” is in fact the banking sector, where it is used to run some ATMs.
TakeDown.NET -> “OS/2”