Next to a solid tripod and ballhead, the accessory every landscape photographer must have is a polarizing filter. Using one will darken the sky (thereby making clouds stand out), will remove reflections from water, plus it will make leaves and foliage appear less shiny. Color saturation is also enhanced. These effects cannot be fashioned with image processing software.
If you want to significantly improve your landscape images, use a polarizing filter!
Polarization displays maximum effect at ninety degrees to the sun. The farther your angle varies from this ninety degree angle the less impact the filter will produce. Because of this, extreme wide angle lenses (and stitched panoramas) often yield uneven polarizing effects across the sky. Also, since this filter blocks light, it will affect your exposure (which isn’t a big problem since most cameras meter the light passing through the lens). I’d recommend a circular polarizer over a linear polarizer.