"Many people don't realise that the UK has tens of thousands of paintings in its publicly funded museums and institutions that are not currently viewable by the public, either because they are in storage or are kept in buildings that are inaccessible to the general public. The vast majority of these paintings have never been published online."
The website aims to have 200,000 paintings (64,000 are online and search-able already!) with tags to help users search by artist, theme or collection.
ps. Don't forget the Google Art Project too!