Having read Eliot Coleman's excellent books on winter gardening in unheated greenhouses (hoop houses), I was eager to give it a try! My first frame (2009-10) was built of PVC pipe, the gray variety sold for electrical conduits that is UV-stable. Unfortunately none of the PVC joints available to buy in these parts are UV-stable; on the contrary they become very brittle very quickly under outdoor conditions. The frame soon became held together at the joints with gobs of clear duct tape! (By the way, clear duct tape is a wonderful thing for working with plastic outdoors.)
The plastic I used was the only kind available in town, 6-mil poly sheeting, not intended for greenhouse use. It lasted pretty well for a year but could not be reused the following year. I built the first year's greenhouse for about $100 -- frame, plastic, and all -- but had to rebuild parts of it a couple times due to wind damage, and I learned a lot in the process. For one thing, I built the structure against the south side of the garage, the only available space. Since that wall has a door, the greenhouse did not need a door; access could be through the garage. That cut down a lot on wind entering the structure and tearing it apart from the inside. However, the greenhouse roof and garage roof sloped toward each other, making a channel where snow accumulated. The first year that wasn't a problem because we hardly got any snow.
I put several full rain barrels in the greenhouse, used them for watering and for thermal mass, and refilled them from the gutter. They froze solid nearly every night but thawed out reliably every day. Surprisingly, this freeze/thaw cycle did not break the plastic spigots as I feared it would.
The second year (2010-11) the frame was much better constructed to withstand the wind, but we got a heavy load of snow that weighed the plastic down to the ground! Most of the photos in the album linked below show the steps involved in putting plastic on that year, since I worked out a method to do it unassisted.
The third year (2011-12) I took out all the PVC and built a much sturdier frame out of wood, but we had such a dry summer that nothing much survived to justify putting plastic on, so I didn't; it first got put to the test in February 2013. In subsequent years I've put plastic on only about half the winters; however, the frame doubles as a trellis for climbing crops, allowing me to make much better use of vertical space.
I experimented with hanging old rain gutters from the garage eave as planters. This worked well in mild weather, but in both extreme heat and cold the plants died.
The greenhouse is the only one of my garden beds where I plant annuals almost exclusively. Perennials take up too much valuable real estate. However, this means I have to pay attention to crop rotation and other inconveniences that perennial beds avoid.
See an album of more photos of this design.