Learning about filters

So, I'm educating myself on different types of filters for my camera.  I already use a filter, specifically to protect my lens, but I wanted to learn how to use them for more artistic purposes.  I now know that if I want to shoot landscapes, I really should own three certain types of filters: circular polarizer, neutral density filter and a graduated filter.  I found some really good examples of how each of these filters were used.  Pretty interesting.  Maybe I'll look into colour filters too soon.

The below picture was taken with a graduated filter, and not by me might I add. Nice shot.