What are some good older PC games?

Hello everyone. I bought the original Unreal game from 1998 today, and I love it. I was wondering if there are any other good older games out there I missed. I was never a big PC gamer until recently, so you could probably mention anything I would've never played it. So don't be shy! Bring on the suggestions! Thanks all. (also, I had the chance to buy Quake II today, but I didn't go for it. Did I miss out?) I'm mainly a fan of FPS's and some RPG's. Not too big a fan of RTS's, though I've heard Diablo 2 is really good. My friend snatched it up before I could.