Circular Polarizing Lens filter, or CPL, is an invaluable tool for landscape photographers. It is one of the rare tools which cannot be replicated in the post processing. It can make the skies bluer, the seas clearer with better contrast.
The way it works is they align the light rays getting through the filter onto the sensor, and can reduce the refraction from the dust particles in the air, and also works on the water reflection. These effects cannot be reproduced by Photoshop. So I highly recommend budding landscape photographers to bring along 1 CPL with you next time you go shoot. Depending on the diameter of your lens and also the quality of the filter, price varies from around HKD400 (USD50) to HKD1200 (USD180) for the top notch ones, e.g. B&W. Try it out and you will never leave home without it.