Polarization is the next category of filter I use a great deal. Some photographers, including my friend Steve Mulligan, never use them. Another successful shooter I know has polarizers stuck on all his lenses. Others, like me, use the filter from time to time. My choice is the warm polarizer made by Tiffen.
Polarization helps saturate colors by removing the sheen of reflective objects.
Just about any surface can be reflective, including sand, snow, rocks, foliage, leaves, Indian ruins, flowers and of course water. The filter also deepens the blue of sky and makes clouds stand out from the background of the atmosphere. With the sun shining directly on the subject, a polarizing filter has no effect and should not be used, but when the sun is at the prime polarization angle of 90 degrees, or to either side of the camera, the magic of polarization occurs. The warm polarizing filter adds a very slight red to the scene, which is an even more pleasing effect. Some polarizers also work well under difficult weather conditions.