If you're out walking on a lovely day - in the mountains or at the beach - and you want to get a fabulous shot of a cloud-filled sky or make the water seem more saturated, then a polarizer is the way to go. This filter is the best way to make the clouds stand out, increase or decrease the saturation of the ocean or increase or decrease the reflection off the water. You can also use this filter when shooting into glass or windows since it will, again, either increase or decrease reflections.
If you're shooting black and white and want to simulate a threatening storm, try throwing on a #25 Red filter. The vista will take on the appearance of an imminent, ominous storm.
Along with a UV or Skylight Filter, a polarizing filter should be a part of everyone's gear bag. It can turn a run-of-the-mill image into a stunning, creative photo by increasing color saturation.
Note! Circular polarizers are specifically designed for use with auto-focus SLR cameras (They will also work on manual systems without problems)