
I too forgot to mention Hurried Descent and Fiend Hunter. I was really disappointed when FH's development stopped, because what had already been done was amazing. Hurried Descent was a great example of an extremely simple idea done really well, which blows a lot of more complex games right out of the water.
I also enjoyed Team Fortress back when I played Quake online and I'm pleased the idea has taken off (now with TF2 on the source engine, though I haven't gotten around to playing yet). Rune Quake and a few others are likewise fond memories.
And I forgot another big one that completely slipped my mind: Token Quake. I really enjoyed coding for and playing that one, and playing against FrikaC and others. It was a lot of fun putting together a character and trying it out in battle.
Lastly, has anyone played Archmage? That was an impressive mod as well, though not among my top 5. I suppose that was the one that really got me started into coding, as I used it for reference regularly for a good long while.
Cheers, all.