The Metropolitan Museum of Art has a newly designed website, and it is absolutely astounding. You can find nearly every piece in their collection, with all the pertinent information listed in detail. Additionally, they have new, large image files of the works, which you can even download (click "fullscreen" and then the little arrow button on the bottom right). I spent a good chunk of time looking through the John Singer Sargent archives (thanks to this post), and was just blown away by his sketches and studies.
Five Figures (from Sketchbook) 1876
Hunting Birds: After the Egyptian Relief Sculpture (ca. 1878)
Alhambra, Granada (1912)