The circular polarizer is definitely the landscape photographer’s best friend. It plays two very important roles. First, it increases contrast between the blue sky and white clouds. Second, it helps saturate the colors in your image. Put these two attributes together with the late afternoon sun and you’ve got a beautiful, warm image to show off.
One thing to note. A good circular polarizer filter is fairly expensive ($75 – $125) but the cheap ones can ruin the quality of your images. I prefer a B&W or Heliopan slim circular polarizer for my EF 17-40mm f/4 L lens since both of these are of excellent design, quality and workmanship. You can find both of these brands at your better camera stores or online at B&H Photo or Adorama. Like all photographic gear, buy the best circular polarizer that you can afford and only buy it once!