A polfilter (pol for polarizing) can give you all kinds of nifty improvements in your outdoor pictures. Here is a little preview. I expect to achieve much more drastic differences come summer:
Left: No filter (or at least filter turned towards “normal”). Right: Filter set to the right angle. Make sure you check out the full-sized version of the photo and pay attention not only to the sky but also to the trees and the windows of the building. Image was sharpened with Gimp btw.