For centuries people around the world have trained trees and other plants to grow into "designed" shapes. The best known example would be the Japanese art of bonsai. Topiary has been done in Western cultures since Roman times. Perhaps the most innovative current practitioners are the Pooktre Tree Shapers of Queensland, who fashion chairs, benchs, picnic tables, and surreal art forms from living trees. Their website is HERE, and THIS LINK has many additional interesting photos.
Addendum April 2009: Wikipedia now has an entry on this subject.