I have posted a fog simulator for HL in downloads. I have always been told that it is not possible to have fog in HL, and the only way to get close is to have water. But I found a way to add it and it works amazingly well.
Here is the link to the discussion page which will take you to the download:
http://www.snarkpit.net/index.php?s=forums&f=7&t=12327