AVL tree applet - Fantastic applet that shows animations of additions of nodes to a balanced, binary tree. Allows you to show weights, thinking, etc.
AVL tree applet - Similar to the previous, but not quite as cute. Still very functional, though.
Binary Search Trees - Explains how binary search trees are structured and organized. Several good illustrations.
Binary Search Trees - Some good diagrams that explain the operations on a BST. Includes links to an applet that you can tinker with.
AVL Tree on JetByte. Gives a good explanation of all four rotations needed in an AVL tree.
AVL Trees: From Software Design Using C++ Includes explanations and som ASCII-art illustrations that show how balancing works in an AVL tree.
AVL Trees from the GNU libavl site. This link is to an online manual which describes a great deal of the terminology and theory involved in AVL trees.
AVL Trees from Data Structures and Algorithms (an on-line reference) - good synopsis with colorful illustrations and terse explanations; short and to the point.
AVL Trees: Tutorial and C++ Implementation humungous page chock full of explanations, ASCII-art illustrations and source code. Also has a cute pic of Calvin & Hobbes at the top.
AVL Tree Algorithm: Shortish page that explains all four rotations with some nice diagrams.