A friend recently wrote and asked, what is the best filter (polarized) for my Nikon D90? What is the difference between filters? My response was this:
There are two types of polarizing filters available — linear or circular. (similar to JPG vs Raw Format) Linear polarizers are more effective and less expensive than circular ones; BUT . . . circular polarizers are needed with just about any camera that has a through-the-lens metering system, or auto-focus.
Polarizing filters exist for most camera types, from medium format to digital point and shoot cameras. Auto-focus SLRs (like the ones I use) need circular polarizers, like mentioned above. In these, the front side rotates which enables the user to see the effect gradually appear in the viewfinder.
There are many manufacturers of filters out there. Personally I prefer actual glass filters as opposed to plastic which means most of the filters I own are either "Tiffen" or "B+W" or "Hoya". Tiffen and Hoya are probably the two biggest names out there (similar to Canon or Nikon) with B+W as a very close 3rd (similar to Pentax). You do NOT have to stick with just one manufacturer, shop around and compare prices.
One Last tip. If you have several different lens filter sizes (52mm, 55mm, 58mm, 62mm, etc) consider buying for the largest size you own and then also purchasing "step-up ring adapters". Step up adapters as the name applies has a small size that fits the actual lens being used and then "Steps up" to the size of filter you own. They are cheap compared to the cost of purchasing every filter 3 or 4 times.