Similar to the ND and GND filters discussed above, it is highly recommended you add a polarizing filter to your photography inventory.  Lee has a very well made polarizer for their full size and Seven5 filter systems.

Circular

Perpendicular to sun, contrary to parallel with sun

Minimizes reflections – glass, water surface

Makes clouds on a blue sky punchy, and brings colors out on flowers or grass by reducing reflections from moisture.

If you capture some polarized images with your camera in landscape orientation, and then switch to portrait, do not forget to rotate your polarizer too.