The built-in fog effect that trueSpace4 uses is simple to use, but doesn't offer much in the way of customization.  wfmm fog adds more parameters, and a method of operation more like older versions of trueSpace - you can set a "near" value and a "far" value, and the background can still remain visible, etc.  The velocity settings allow you to create a rolling fog effect in an animation.

Examples:

bloom

postgame