Splay tree
“All normal operations on a binary search tree are combined with one basic operation, called splaying. Splaying the tree for a certain element rearranges the tree so that the element is placed at the root of the tree.
A top-down algorithm can combine the search and the tree reorganization into a single phase.”
http://en.wikipedia.org/wiki/Splay_tree
The splay tree modifies itself every time it is searched becoming more and more efficiently organized over time.