IBM OS/2 Warp
- where the whole Windows thing started. I worked on this for quite a few years in Boca Raton Fla then later in Austin Tx. A great OS that introduced things like HPFS, the predecessor to NTFS, multiple windows, drag and drop.
It still has an active community and is in use in some corporations. I've often thought of installing it as a Client under Oracle VitrualBox but can't get motivated to do so. I still have the install CD (and 1.44Mb diskettes) around here somewhere.