This is off topic, but since it also concerns one of my maps, it will be fine to post it here.
A while ago, I read a comment somewhere of a Gmod user loading the Fallingwater map and trying different fog combinations. I finally decided (and learnt how) to see it for myself, and I took this screenshot.
http://img152.imageshack.us/img152/4497/creepyeo4.jpg
In my opinion, it looks quite creepy!
The interesting part came when I decided to noclip and fly around. Since fog also works in the Z-axis, there was a very interesting visual effect of creepy HL2 trees emerging from the fog, while the ground stayed invisible. I think it might look nice in horror style maps. A scene of a narrow railway over this foggy forest sprang to my mind... I think people should experiment more with fog in custom maps. At least, that's what I plan to do in the future!