Or Polarizing if you can't spell properly ;-)
Tied in with this entry - Little Fawn - where I was shooting some water deep down in the Queen Elizabeth Forest Park, these two images show quite clearly the effect of using this filter on the same subject. Even though it was overcast and the location was shaded there's a pronounced difference between the polarised and non polarised versions.
I always have one packed - currently using a Hama Circular Polarizer - with a selection of adapter rings to fit it to a range of different lenses (large 72mm filter that's used on a selection of smaller diameter lenses - a LOT cheaper than buying filters for each).
Both images shot with the following settings:
WB - 7500 (Shade)
Click on each image for a full screen version