OS2 Warp Family
Here's a major alternative Operating System that Microsoft illegally tried to crush (in the United States vs. Microsoft antitrust suit, it was revealed that Microsoft prohibited software companies from using Microsoft programming tools to build OS/2 apps, and also forced hardware makers to pay Microsoft a penalizing fee if they installed OS/2 instead of Windows).
Warp was planned as a graphical, multitasking, fully compatible successor to DOS (a "better DOS than DOS"), and in that capacity it succeeded completely. OS/2 was originally a joint programing effort between Microsoft and IBM, and after they split (prompted somewhat by Microsofts unwillingness to further support DOS), core portions of its code was incorporated by Microsoft into their competitor 32-bit OS, WinNT (and its direct successor, Windows 2000).
The Warp family of IBM PC operating systems began with OS/2 version 3. To encourage usage, IBM once gave away Warp 4 for free, and the Australian Personal Computer magazine even bundled a Warp 4 CD along with its July 2000 issue. The stability of the software can be extremely high, and mission critical business applications such as bank ATMs, supermarket checkouts and subway ticketing machines, still rely on Warp to get the job done.
NOTE: With the exception of a few DOS utilities that perform low-level manipulation of the file system, DOS software will run correctly with Warp.