Polarising filters are incredibly useful bits of kit to have in your camera bag. They can be used to create a number of effects but are most commonly used to control reflections on shiny surfaces such as water of glass. Most useful for my however is the ability to create contrast between the white clouds and a blue sky. You can create some beautiful dark blues using a polariser and it is an easy way to enhance your landscape photography. It is also difficult to replicate the effects of a polarising filter in photoshop especially when dealing with reflections on water.
I have experimented with several makes of polarising filter over the years and have settled on a B&W for its optical quality and solid construction. A quick word of caution before buying a filter, make sure that you get a circular polarising filter not a linear one as most cameras metering / focusing mechanisms may be affected by a linear one. If in doubt, check the manufacturers guidelines.