One reason the polarizer is so useful is that what it does is next to impossible to do in Photoshop: It suppresses unwanted reflections off of shiny objects, allowing the texture of the object itself to be seen more clearly. This can be particularly striking in making the surface of water disappear, revealing the fish, etc. below. Another one that deserves mention, and is practically indispensible for shooting landscapes, is the GRADUATED NEUTRAL DENSITY filter (Singh-Ray is the best). This darkens the sky about 2 stops (depending on which one you use), so that you not only get good color saturation in the sky (rather than burnt-out white), but also good details in the (unfiltered) shaded areas. The best landscape photog I know repeated claims that he "Lives and dies by" this filter.