Utilize. In the strictest sense, utilize means to make the best of something not meant for the job -- he utilized a coat hanger to repair the car. It can also be used to mean making the most practical use of something -- utilize every square inch of the land. In all other senses use is better.
Venal, venial. Venial, from the Latin venialis (forgivable) means excusable. Venal comes from the Latin venalis (for sale) and means something that is corruptible (capable of being bought).
Viable does not mean feasible or workable or promising, as is often thought. It means capable of independent existence.
Wean means to be parted from something no longer needed, as babies are weaned from their mothers' milk. Often the word is used wrongly to mean "raised" (Today's teenagers weaned on rap music...). If you are not following wean with from you are doing something wrong.
Weather conditions, weather activity -- just stick with weather.
Whether or not. The second two words should be dropped if the phrase is being used to mean if.
Whet one's appetite. Not wet. Whet has nothing to do with increased salivation; it means sharpen, and is the root of the word whetstone, used to sharpen knives.
Wound, scar. Not interchangeable. The scar is what is left behind after a wound heals. It is wrong, even figuratively, to speak about a scar healing.
Zoom. Stricltly speaking, zoom should be used only do describe a steep upward motion, and not just speed. It is too late to do anything about zoom lenses, and probably cars zooming around a track, but avoid using the word for fast downward movement (the plane zoomed down on the city), especially when swoop is available.
And that's it. I have more grammar books around here -- should I continue to find stuff in those books to blog about?