What about the database that started it all in the PC world, Ashton Tate's dBase III? Through all it's different disguises - dBase IV, Clipper, FoxPro, etc - it ran the first mission critical business applications for the small, medium and even some large companies, many of which are still running a decade or so on.

dBase? Well, I did mention FoxPro...

I have an issue with dBase, since, iirc, it was the first database to seriously attempt to devalue the term "relational database"....