Explore museums from around the world, discover and view hundreds of artworks at incredible zoom levels, and even create and share your own collection of masterpieces.
Yes, this is Google, being evil once again. Seems as though it's just getting going (or so one hopes), but you can already see a bunch of work from various famous museums and do a sort of walk-through of these museums. If you've used Street View in Google Maps, it will feel familiar.
(I've just started looking around.)
[Added] Google's plain-text intro/guide to the project.