In Madrid, Spain they currently building a huge structure called an “Air Tree” or “Eco Boulevard de Vallecas”. The Tree was created by Urban Ecosystem to be a social center, and to improve the surrounding environment. The structure is also completely self-sufficient, generating all its own power with solar cells. Any surplus energy is sold to the electrical grid. It also produces oxygen using its arrays of plants and trees, hence the “tree” appellation.
I'm going to lift this one straight from Metaefficient, where you can read more:
at 8:58 AM