In the July 2006 edition of Consumer Reports several different car (or truck) waxes were tested. In the Liquid wax category BLACK MAGIC Wet Shine Liquid Wax ($7 a bottle) was rated the best along with TURTLE WAX Carnauba Car Wax ($7 a bottle).
What was interesting is that these readily available (i.e. local auto parts store or Wally world) waxes outperformed much more expensive waxes like Zymol (what I use) and Griot's Garage (what some TS'ers sware by). I have posted a scan of the rankings at: http://www.geocities.com/sawyerkt/wax.jpg