Google Arts & Culture have done a heroic effort in uploading collections from around the world to their platform. It is a great academic tool though the ‘visitor experience’ needs to be polished up a bit. Of the museums who signed up to the service, you can see the MoMa in New York, The National Gallery in London, and the Acropolis Museum in Greece, to name but a few. I am adding a link as it is interesting but it hardly competes with some of the other recommended online tours.
See more here.