OK, this one is just fun.
It’s called The Google Art Project. You can see paintings and artwork from some of the world’s greatest museums (The Uffizi, The Hermitage, The Metropolitan Museum of Art in New York, among others).
Zoom in on a particular section in high-definition, then wander along the corridors, like a real tourist. Or, search for a specific work, or a particular museum.
It’s a purple cow (in the Seth Godin sense) and an ideavirus (Seth again). It’s remarkable and it will spread. Heck, if any Chagall paintings are included, there probably is an actual purple cow.