Gobe (now known as URTH) have created a reliable Circular Polariser (CPL) filter in a range of sizes.
CPL filters add more saturation and vibrancy to your pictures without you having to adjust your exposure settings, meaning you can get some more motion in your images but not overexpose, or do low aperture portraits without making things too bright.
Basically, its like putting sunglasses on your camera!
For an example of what this looks like, click here
Available in multiple sizes 49mm, 52mm, 58mm, and 62mm