ArtUK.org

Art UK is the online home for every public art collection in the UK. They are a charity (so do donate), and their website represents a collaboration between over 3,200 British institutions. These are public collections. They are owned by the British Public (technically) so now, with the efforts of Artuk.org, you can enjoy the art. It is bursting with interesting facts, articles and opinions, and is a sure way to discover something new.

See more here.