The Google Cultural Institute is something many people have never heard of but is an amazing educational resource. It is a web-based resource that covers Art, History and other culturally significant artefacts and ideas from museums and galleries across the globe. Read about the Google Cultural Institute here and access resources for History and Geography teachers here.
Read a 2013 Guardian interview with the Director of the Cultural Institute here.
The difference between this and a lot of other websites is that every single image is in extremely high resolution, as well as enriched with a whole host of tools and meta data. For example you look at a museum's Vermeer, zoom into it, compare it with another one, and then take a virtual walk through the museum by using our street view technology. (Amit Sood, from Guardian Interview)
If you have never visited, I would heartily recommend putting aside a bit of time and treating yourself to a tour. There is help and a video guide built in to get you started.