Here are some of my favorite teaching links!
The Incredible Art Department is a great resource for art projects or project ideas for all age groups, also many lesson plans are attached along with curriculum standards.
Art Projects for Kids is another great resource for projects, but only for the Kindergarten to Fifth grade age groups.
Art History Resource is a perfect go to website for any research needed for project ideas or lessons.
The National Gallery of Art has a kids art zone, specifically aimed for young students who are interested in art and art history. They also have an interactive website where students can create art online.
The Museum of Online Museums is a great place to look for examples of different styles of art or famous artwork. It has a permanent section of artworks that never rotate, but there is also a section of continuously changing artwork similar to what you would find in a gallery.