Which is very fitting considering the link I found is the online museum of all online museums.
The Museum of Online Museums, or MoOM, is an ongoing experiment of web publishing, design and commerce from the Chicago based design company, Coudal Partners.
The MoOM is updated often and they have a rotating gallery. There probably isn't a museum you can think of which isn't listed.
Online museums are a good way to bring otherwise inaccessible art to the classroom. Often times the museum will allow you to zoom in and navigate each piece and the good ones will have a background of the piece to help set the scene.