Microsoft's OS/2

That would generally be OS/2 <= v1.3 which, it should be noted, will fail to work in 99.99% of virtual machines. Whilst OS/2 v2+ uses some odd features that make it difficult for it to work in some virtual machines, OS/2 <=v1.3 is downright hostile - it needs patching to work on real modern hardware anyway (PCI? What's that?).

Technically, there was a number of disk sets of Microsoft OS/2 2.0 beta code, created some time before the Microsoft/IBM split was finalised. These disks are incredibly rare.

Still, 'Microsoft OS/2' can only reasonably be called <=v1.3. It can't be denied they contributed a fair bit of code, although arguments rage over the quality of their code to this day.