Just over a year ago we reported on the Google Art Project, which at the time had around 1,000 works from 17 museums. According to the Google blog there are now over 30,000 high resolution works, from museums all over the world. They also have 360-degree panoramic views of gallery interiors, and a growing number of gigapixel images (photographed in super high resolution, allowing for extreme detail viewing). In addition to expanding the holdings, Google has improved the search and access capabilities, allowing users to build personal galleries and register comments.
You can watch demo videos on the blog site, or go straight to Google Art Project here.