I've always loved old wooden framed windows and have been collecting discarded ones from building sites/streets for the best part of a decade. When I took on my allotment a couple of years ago I decided I wanted more covered growing space and the windows came into their own. It doubles up as a potting shed and tool storage space.
It was built on a budget; the main frame was made out of new tanalised wood but most of the other wood and materials were waste from gardens I've worked in (I'm a gardener/landscaper by trade) or from the skips of West Yorkshire.