I just purchased a new filter for my Nikon D80 from a great NYC company called Adorama. Read my review of this product below and learn a bit about how it works.
Nikon 52mm Circular Polarizer II Thin Ring Multi-Coated Glass Filter
Great product!michaelgc Bellingham, Washington 2/21/2009
Pros: Easy To Use, Clear Optics, Firm Mount, Strong Construction
Best Uses: Lens Protection, Color Correction
Describe Yourself: Photo Enthusiast
Bought this filter after spending days agonizing over which polarizing filter to buy and where to buy it. I'm really glad I gave Adorama a try. Very professional salesperson, didn't try to sell me more stuff to pad the order. Friendly, quick process. The order got all the way to me in Washington state in only a few days.
As for the filter, it came down to economics and my personal level of sophistication with all things photographic. I love taking photos, trying to be artsy while doing it and I love using technology to manipulate my pictures to make them better using Photoshop. I do like how the filter makes otherwise dull skies pop and water seem more translucent. I am sure there will be many more opportunities for me, through experimentation and reading how others use this filter, to make my photos even better.