A Lending Library. You choose the items (DVDs, slides, teacher packets) and the National Gallery of Art will loan them out to you, free, for a certain length of time, like a library would. They'll ship the items right to your door! Your only cost is the return postage (this is open to all teachers, including homeschooling teachers).
Interactive lessons which include lesson plans and printables for several subjects (you will find these lessons listed at the top of the home page, and more if you scroll down to "super-learner interactives")
My favorite section of the site for teaching is the search feature, which allows the educator to search through the National Gallery of Art's vast collection by curriculum, topic, or artist. This is also accessed on the home page.
And, finally, set the kids loose at NGA for Kids, a kid-friendly site with more art-related features, games and interactive fun than I could tell you about in this little paragraph. It's so big that I should highlight the NGA for Kids site on its own someday.