The Utopianist is an on-line magazine about the future. Articles cover technology, culture, economics, environment and even music. There is plenty about sustainability, although they generally avoid the term and lots about design.
Check out this article about a subterranean, solar-lit NYC park. If you watch the video, the architects have invented a clever device to channel sunlight underground so they can have grass and trees.
This site is about a positive future and what we can do to get there. I think you'll find it interesting.