The most useful filters for digital SLRs
The filters you really need are polarizer filters and graduated neutral density filters. The effects of these filters can not be done by the use of photoshop. For interesting discussions on photo.net, please read here.
Polarizer filters are the most useful filters which can add ” wow” factors on your photos.
These filters can cut the polarized light hitting on the lenses. The amount of reduction of the polarized light is adjusted by turning the polarizer filter. The effect is most noticeable at an angle of 90 degrees from the sun. They can saturate colors and provide better contrast. The polarizers can also remove unwanted reflections from non-metallic surfaces such as water or glass. Here is the link to the best polarizer filters:
Graduated Neutral Density filters:
Sometimes it is necessary to balance the light intensity in one part of a scene with another. For example, in bright contrasty landscapes. In these case, a graduated neutral density filter is used. The neutral density filters are gray toned in one half of the filter and are smoothly graduated into the clear portion. The best neutral density filters are B&W Graduated Neutral Density filters which are available in 2 grades; #501 (approx. 1 stop density at the top) and #502 (approx. 2 stops density at the top). Here is the link of B+W 77 mm Graduated Neutral Density filter.