While many old school film filters are now obsolete in the world of digital photography, a few remain very useful. One of these is the circular polarizer filter. However, you need to know how to use it to get the best out of it!
What does a polarizer do? Put simply, it reduces the amount of reflected light that goes to your camera's sensor. Blue skies appear a deeper blue, and reflections from water can be removed entirely.
Polarization is at its most effective at 90 degrees to the sun, such as when your subject is at a right angle to the sun. At 180 degrees, with the sun behind you, polarization will be non-existent. Circular polarizing filters come with two rings, and you simply twist the front ring to activate polarization. You'll know when you've achieved polarization by reflections disappearing, or when the contrast between blue sky and clouds increases.
Bear in mind that using a polarizing filter will reduce the amount of light reaching the camera's sensor by as much as two or three f-stops, so you'll need to adjust for this. It's best to set your focus before putting the filter on, or switching to manual with it on as the camera's autofocus system will find it too dark to automatically focus. This is also essential with cheaper camera lenses that have a rotating front element, as this would change your polarization by rotating the filter as it focuses.