Interplay, Microprobe, Sierra On-Line, Bullfrog, Dynamix, Origin, all long gone.
Activision is still around, but it's something completely different. Same with Atari (although theres a nostalgia brand now, so maybe back).
Of them all, I think is have to say I'm most nostalgic for Sierra On-Line, although Origin gives them a run for most nostalgic.