The Metropolitan Museum of Art, NYC

The one and only.  Their collection database isn't the easiest to navigate, but the images are worth it. Here are some ready-made searches of their holdings: Caravaggio   Rembrandt   Vermeer

MoMA, NYC

The Museum of Modern Art has over 23,000 objects in their online collection alone--this is a great resource.  Plus, their online store has some of the coolest stuff anywhere.

Getty Museum, Los Angeles

The Getty focuses on Western art from the Middle Ages to the present.  Their online database is easy to use and a good general resource.

National Gallery, Washington, DC

This site has everything from web tours to streaming slideshows, plus a fully searchable online collection.

Philadelphia Museum of Art

Tate Museum, London

Guggenheim, NYC, Bilbao, Berlin, Abu Dhabi

Their online collection is comparatively small--only about 700 objects--but still worth a visit.

The Rijksmuseum, Amsterdam.

Has some of the most wonderfully large and detailed photos of art on the web. They are much more generous with their images than most other museums--you can download great desktop pix. Lots of Rembrandt and Vermeer, but also scads of other great painters, like Jan Steen, a personal favorite (-st). Here is his self-portrait

The Prado on Google Earth. Amazing super-high quality reproductions. But you have to download Google Earth to use it.