Scientists at Tel Aviv University are developing methods of growing trees such that the roots form stable structures that have many uses, even as living homes. According to aftau.org, "the original 'root-breaking' research was conducted at the Sarah Racine Root Research Laboratory, the first and largest aeroponics lab in the world."
Aeroponics is similar to the much more familiar hydroponics, except that the technique allows plants to be grown with the roots suspended in the air instead of in nutrient rich water. For the "geeks" out there (including myself!), both of these techniques are showcased very well at Walt Disney World's Epcot attraction, Living With The Land. That is kind of ironic since most of the plants throughout the ride are grown without soil! But I digress..
Pilot projects are currently in progress in Israel, Australia, and the United States, and include "growing" park benches, playground equipment, and even street lamps.