eComStation
OS/2 was an operating system originally created by Microsoft and IBM. IBM continued development after Microsoft left it and OS/2 competed with MS-DOS and the original versions of Windows. Microsoft eventually won, but there are still old ATMs, PCs, and other systems using OS/2. IBM once marketed this operating system as OS/2 Warp, so you may know it by that name.
IBM no longer develops OS/2, but a company named Serenity Systems has the rights to continue distributing it. They call their operating system eComStation. It’s based on IBM’s OS/2 and adds additional applications, drivers, and other enhancements.
This is the only paid operating system on this list aside from Mac OS X. You can still download a free demo CD to check it out.