Artchive.com used to be one of my favorite websites. They have a great image viewing tool that allows you to zoom in on fine details on thousands upon thousands of classic paintings. Also, the written descriptions of artists are quite good. However, the site has sadly become littered with advertisements including pop ups and interstitial page ads. I would gladly pay a few dollars to make these disappear.
My favorite artist of the day - Klimt protege Egon Schiele. Check him out:
http://www.artchive.com/artchive/S/schiele.html