These days, you can take a virtual tour of paintings at the MoMA, Met, Uffizi Gallery, Hermitage, Rijksmuseum, and National Gallery and other major museums, thanks to Google's Art Project. And don't forget the Sistine Chapel and the Dead Sea Scrolls.
Now let's add one more to the list -- a panoramic virtual tour of the Smithsonian National Museum of Natural History. You can visit the whole museum on your own, walking from room to room, checking out fossils of countless dinosaurs, specimens of early sea life, exhibits on the ice age, and much more. Begin the general tour here, or find a more targeted area of interest here.
Note: the tour requires Adobe Flash Player, version 9.0.28 or later.