System Shock 2 and Mechwarrior 3 just missed the cutoff (released in 1999).
Morrowind was indubitably one of the best open-world games I've played. I remember wandering somewhere at night, and seeing the twin moons rise and being struck by how damn pretty it looked. If I didn't think Bethesda would fuck it up, I'd beg them for a graphical remaster.
While the original campaign of Neverwinter Nights was... less than ideal, the Aurora Engine's capabilities led to some absolutely fascinating stuff, like entire fan campaigns and even quasi-MMO settings.