Greenhouses made from discarded old windows -- wonderful reuse of building materials, additionally old windows are surprisingly easy to come by certain times of the year making this lovely DIY backyard addition relatively inexpensively.
Soda bottle green house - I've been looking for a way to do this. I have trouble with plants and I'm trying to grow my own spaghetti squash. It is just so expensive in the stores. It's squash, for goodness sakes!