Circular Polarizer Filter

Water Lilly with Circular Polarizer

A Circular Polarizer (CP) filter is the only filter you absolutely need for outdoor photography.  Sure there are others that can be helpful in certain situations, but a CP filter is essential gear.  They are extremely beneficial for cutting down on reflections on leaves and flowers, water, and anything else that reflects light.  This allows you to take photographs with more saturated colors, and the difference can be rather striking.  CP filters are also used to make the sky a deeper blue, though in my opinion, this effect is often overdone.  There's no way to duplicate the effect of a CP filter in Photoshop, so you have to use it in the field.  

Baltimore's Inner Harbor with CP Filter

There are a few things you should know to make the most of this filter.

1. Rotating Filter. The filter works by selecting directional of light--this means that the effect of the filter will change as you rotate it on the front of your lens.  After screwing on the filter, you will be able to continue rotating the filter.  You'll see reflections disappear and appear, and correspondingly, you'll see the colors saturate and desaturate.  A word of advice: always rotate it in the same direction that you screwed it on, so that you won't accidentally unscrew it and drop it into a stream (learn from my experience).
2. Effectiveness.  The filter is most effective when used at 90 degree angles to the sun.  An easy trick is to point to the sun with your index finger while sticking out your thumb at right angles to your finger.  As you rotate your hand pointing to the sun, your thumb will show you directions where your filter will be most effective.
3. When to Use. When the sun is out, you probably want to use it, but I find it helps on cloudy days as well.  The  easiest way to check is to pull it out and look through it.  Rotate the filter in your hand and see if the colors change.  If they do, you probably want to use the filter.
4. Effect on Shutter Speed.  The only exception to that rule (for me) is when fast shutter speeds are necessary.  These filters can cut out 1-1/2 stops of light, so I rarely use this filter when shooting birds.  I want the image sharp.
5. Over-Polarizing.  It's possible to over-polarize an image.  For instance, I like to cut down on the reflections in running water, but if I remove all the reflections, the water is nearly invisible, and it doesn't look natural.  So I back off a little to give the photograph a more natural look.  Sometimes the dark blue skies produced by a CP filter can look unnatural as well.  The photograph of the water lilly above retains the reflections of reeds.  If I had rotated the CP filter differently, I would likely have eliminated even those reflections, but they were part of my interest in the scene.
6. Wide-Angle Lenses.  When shooting images with wide angle lenses, the lens often will take in an expansive scene--so much so that the filter will polarize the scene unevenly.  For instance, wide angle shots that include a blue sky will show the sky fade darker and lighter, with the darkest part of the sky being in the direction 90 degrees from the sun.  So be careful when using this filter with a wide angle lens.  You can often minimize this by using a focal length longer than 35mm.

Baltimore's Inner Harbor showing Uneven Effects of CP Filter