Thematic essays Searchable by category (from Heilbrunn Timeline of Art History)
for example:

Essays about modern and contemporary art
Abstract expressionism
African influences in modern art
Costume in the Metropolitan Museum of Art
Art and photography 1990 - the present
Anselm Kiefer
and many more.

Smarthistory Online Art History resource
Art history resources on the web
Art in the picture An introduction to art history (Artists, paintings, styles and movements, videos)

Art History Resources
ArtsourceLists bibliographies, historical sites and art and architecture programs and more.
ArtcyclopediaAims to be a definitive guide to museum-quality art on the Internet.

Internet ArtResources
Includes gallery walk, studio visits, art schools, etc.
Mother of All Art HistoryCompiled especially for art historians. Well maintained.
Voice of the Shuttle: Art and Art History PageIncludes general resources, museums, galleries, auctions, artists, etc.

World Wide Arts Resources
A general arts information site. Quite commercial.
Bayeux Tapestry (plates) from a book by Charles Stothard 1822