OS/2
IBM's OS/2 Warp is one branch of what was originally a joint development between Microsoft and IBM, the other branch being known today as Microsoft Windows NT. It can run most DOS and Win 3.x programs seamlessly, as well as some Win9x software.While not abandoned by IBM, it is not very actively marketed by them anymore. However, a company has licensed the right to market an up-to-date OEM version of OS/2: eComStation.
Also, OS/2 has long been actively supported by a legion of developer-users and enthusiasts, and good development information is available on the Team OS/2 and ("Seattle OS/2 users group" | http://www.seaos2.org/refs/develop.html) MIA sites.
FTP sites for OS/2 software: Hobbes, ("FTP-OS/2" | ftp://ftp-os2.cdrom.com) MIA
UPDATE:
IBM has announced that OS/2 is now truly dead, about 2 weeks after M$ declared that MS-DOS, Win 3.X, Win 95, and Win NT 3.X dead, on 01/01/03. OS/2, you may recall, is WinNT's not-so-evil twin.
This page is linked from: Microsoft Windows