The National Gallery of Art Online

The National Gallery (the US one :D) has decided to scan and upload 25,000 of it's holdings to the web, and put in on a website where you can view and store favorites in a lightbox, or download various resolutions for lectures... or apparently "whatever". A quick search for "Winslow Homer" pulled up not only his better-known works but far more interesting sketches and some engravings that were done based on his paintings--things you would only see displayed as a "special exhibit." Navigation is a little clunky but it's not horrible, either--you probably should pull down the information under the magnifying glass icon before you pull the main image, and rename both files to match, and to be something you can remember--they are currently file named with letter/number catalog designations. This is great for the museum and not too useful when you are dragging through your local Downloads. This is what the information popup looks like:

Anyway, here is where you go to see it all: