Circular Polarizing Filters

Normally I don't carry lots of "extra" equipment beyond basic Camera/Lens, but there is one case where I always bring something extra . . .
a Circular Polarizing Filter for Car Shows.

So Many Reflective Surfaces.  Shiny body paint, windows, and chrome.

Here is where a Circ-Polar filter really shines.  (Or doesn't "shine" . . . your choice.)

Same view of same car, with the Circ-Polar filter "twirled around" to eliminate the glare/reflection (mostly from the sky).

Red Black Car wo/Sky ReflectionRed Black Car w/Sky Reflection

Look how much deeper and richer the reds and blacks are in the left image.
The right image has reflections from the sky and is "greying" or "blanding" the overall color contrast.
The right image just looks . . . "dull".

Now look at the (left on the screen) fender.  Yes, on the Left image, you can see more reflections of cars/people from outside of the frame on the left.
So, a circ-polar filter doesn't necessarily eliminate all reflections, but it can change the DIRECTION of where light is allowed to come from, and radically orient what you see.

By killing the glare from the sky, the colors on the left are richer and brighter.  Contrast is improved.

Here's a (handheld) video example of three cars while twirling the circular filter around.  
(When twirling with your finger, try and twirl the front element of the lens-pair in the direction of "screwing on",
  so you don't unintentionally unscrew the filter, while trying to adjust.)

I pretty much only use the CircPolar filter for Car Shows.  But remember: "Every Blade of Grass in a Field . . . is a Reflective Surface".  
There are circumstances where you can get much deeper-richer greens, if you use a Circ-Polar filter, even on non-obvious things like landscapes.

It's also awesome for things like "Barber Shop" windows.  Do you want to see the Reflection in the window?  Or the people INSIDE the shop?
With a Circular Polarizing Filter . . . that choice is usually up to you, not circumstance.