Slippery Surfaces
Want to make
your player feel uneasy on the feet then eh? Easiest way to do that is to
apply a func_friction to your level.
Make a block where you want the slippery area to be, I recommend clipping
somewhat to make it look realistic, don't leave it square.
Once you have
made the block tie it to entity (CTRL+T) and choose func_friction from
the list.

In the percentage field the lower the number then the more slippery it will
be. Also if your making it look like water or ice don't forget to change the
Render Mode and amount, bearing in mind that 0 is invisible and 255 opaque.
That's all there is to it, amazingly simple.
By Blink