the 2 mods mentioned (Doom 3 mars city and Half-life 2) are really out there.
Willettfx's monster mod is also a very complex mod.
Here's a couple mods that caught my eye.
1- H2O reactor at http://www.melloman.li/
The site is in lithuania (I think)
You have to click "projeckte" and then click "H2O reactor"
The site wasn't very far last I looked but the machine looked very futuristic.
2 - KANA
3- Crazybillybob's r2d2 mod
heck there are so many how can you pick just one?