The 105mm Circular Polarizer from Hitech is one of the two most important filters (along with a UV filter) that a photographer can keep in his gear bag. A polarizing filter can change a run-of-the-mill shot into a "keeper". A marginal, bland sky can suddenly be transformed into a brilliant blue with clouds that jump off the image and show great separation from their blue background.
If shooting into water, hot spots off the surface can either be accentuated or minimized. However, a polarizer's primary use is to make a blue sky bluer, enhance clouds and kill reflections when shooting into glass or bright metal. As an added effect, 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.
The Hitech filter system is made by Formatt - an English company that is renowned for its high manufacturing standards and consistency of quality. Their CR39 resin is manufactured in large sheets that are oven cured. Their exclusive Absorbing Cell Technology absorbs color as opposed to other filters that are coated. Once cured, they are sliced up to create individual filters. In this way, maximum consistency is maintained from filter to filter. This becomes crucial when you wish to change filters either mid shoot or from day to day.