The folks at http://www.haltadefinizione.com/ have scanned masterpieces by Leonardo da Vinci, Caravaggio, Botticelli and others at incredibly high resolutions. You can zoom in to see the finest details of the brush strokes. You can even see any cracks that may have appeared in the paint. This is likely more detail than you would be able to see in person without magnification.
I think we can learn a lot from studying the works of past masters. Have a look and let me know what you think.