Rabbit Hole is a singleplayer mod settled in a surreal place. You start on the surface of the normal city and then get down to its undergrounds. And the deeper you get, the more weird things you discover. The mod is focused on massive combat and horror atmosphere. The gameplay is more quake-ish than classic half-life style, featuring massive zombie arenas and some experimental gameplay sections.
The mod starts out kinda average, but suddenly becomes really great.
The later part kinda reminds me of Serious Sam :)