Theoretical Paradigms Outlines of key ideas and links. Lots of material, some areas more fleshed-out than others. Timelines and definitions of terms.
Material on Theory by Professor John Lye at Brock University. Clear and readable outlines posted as course materials.
Background Materials by Professor David Arnason at the University of Manitoba. Summary articles, lots of content, though not the smoothest reading, and some dense typography and layout. Good links to other resources at Arnason's home page.
Our Postmodern Life Pretty pictures (like the one below), personal views but, hey, readable text.
Words of Art is a glossary of terms, remarkable in its coverage. Definitions are short, but densely crosslinked.
Architecture on the Web is a comprehensive set of links to architecture resources.
The Web Gallery of Art "is a virtual museum and searchable database of European painting and sculpture of the Gothic, Renaissance and Baroque periods (1150-1750), currently containing over 8,000 reproductions."
The WebMuseum has a wide-ranging collection of "famous paintings", including 20th-century works.
The Electric Gallery Good reproductions of work by present-day painters, many for sale.
(If you know of a gallery of recent "name" art, please mail me.)