The Incredible Art Department page is loaded with art lesson ideas. Apart from the vast amount of art ideas, you can also sort your search by medium or grade level. This is a fantastic resource.
Art History Resources is another incredible source of information. On this page you can link to other web pages based on the era of art history that you are looking for.
Arty Factory is a great resource for lesson plans, design lessons and art movements.
Along with these pages, I have often turned to wikipedia as a source of information on different elements and principles of design, art movements and definitions of art terms.