Let see, Hardware companies I miss: Quantum, their fireball series of drives were top notch back in the day up till they were bought my maxtor. In fact a 10 year old 40gb quantum fireball drive, I had bought back in 98, is still alive and kicking in a low end computer I put together for a friends kids to use.
3Dfx, my first real 3d card was a diamond Monster 3d, literally ran it till it wouldn't work any more, which sadly was only a couple years. Also owned a Voodoo 2(back when Creative labs made 3d cards), Voodoo Banshee, Voodoo 3, and Voodoo 5 5500 card at different intervals.
Gainward, I agree with others on this one. Believe it was the Geforce 2 Ti 450 that I had that was made by them. Had that ah heck for years, right up until shortly after I got ahold of the GeforceFX 5600ultra. Then it went into a friends computer and kept on truckin for a while longer.
STB, back in the day before 3dfx bought em up, they made some of the best, and best supported voodoo, and voodoo 2 cards with their Black Magic series. Also one of the earliest companies to stylize the black PCB with their products.
Software:
Interplay, made some really good memorable games. Descent, Descent 2, Descent Freespace 1 and 2. Starfleet academy. And along with Bioware they released Baldurs Gate PC version.
Parallax Studios, the main company that designed many of the interplay games. Same for Black Isle Studios.
WestWood Studios, what can I say..... Some of the earliest games I played on Atari/Commodore were by westwood, and almost all their games were good. Not to mention they brought us C&C.
SSI, and Microprose, what ever happened to these 2? They use to make some decent games and other programs.
Blizzard North.... The diablo games just aren't gonna be the same without that team on em.