Polarizing filter for the landscape photographer is a must have. A polarizing filter serves a couple purposes but the main reasons I use one in most of my landscape photographs are:
1) It removes reflections from surfaces that reflect light such as water or metal. When you are photographing lakes or streams you will often run into lots of light reflections coming off the surface of the water. More often than not this is an undesirable effect.
2) It adds saturation to the colors in the photograph. I used a polarizing filter for this weeks post picture. By cutting down atmosphere reflections it adds pop and contrast to the blue sky which in turn provides more definition to the clouds. Its also a great tool for fall photographs. If you want your fall colors to be rich with orange, yellow, and red, try putting a polarizer on your lens.
3) Polarizing filters also reduce the amount of light entering the camera. This could be bad or good depending on your situation, but 95% of the time I'm shooting from a tripod so this doesn't bother me, it actually allows me to obtain a longer shutter speed. Great for waterfalls, rivers, and streams.
I encourage you to buy a good polarizing filter. Buying a lens for 2 grand just to put a $20 filter on it doesn't make sens to me. Check out B+W for top of the line filters, or if you are on a budget tiffen makes some reasonable filters as well. B+W 77mm Kaeseman Circular Polarizing Multi-Resistant Coating (MRC) (Slim) Filter. Make sure you find the correct filter size for the lens you are using. Also if you are shooting with a wide angle lens you will want the "slim" filter. The link above is for a 77mm which is pretty popular size.
This weeks post picture was taken about 1 hour before sunset using a polarizing filter, shot at 16mm f16 ISO 100.