Never leave home without it..

Here’s another before/after example illustrating the benefit of using a circular polariser:

Move the slider to the right to reveal the ‘after’ image with the polariser..

Note, other than the crop and adding the copyright text, these images are both straight out of camera. The only difference between the two was the use of the polariser.

If you’re looking to buy a polariser, in my opinion you can’t beat the ones made by B+W. I use their top-end ‘Kaesmann’ filter, which isn’t cheap but it really is the Rolls Royce of polarisers. Here are a couple of recommendations for you, with Amazon Links:

Hoya Pro-1 (minimum spec I’d recommend for beginners)
Hoya HD Digital Circular Polarizing Filter (better)
B + W Circular Polarizer Filter With Multi Resistant Coating (pro-spec)
B+W Kaesemann (The best imho!)