Real trees might trump fake trees in being green, but nothing beats building a Christmas tree from recycled bottles. These wonderful recycled Christmas trees are popping up in Paris and you can build one yourself.
The bigger versions in Paris are actually a promotion by DesignPack Gallery to push these home models out (and to spread the word on recycling, of course). The message is to collect bottles and re-use them in unique ways, like making Christmas trees.
I think they look great and all you need is cardboard to build it! And, well, a hundred bottles of Sprite. [DesignPack Gallery via Recyclart via Notcot]