Best thing for making a cold frame.
I had one that was made from an old window at my old house and looked kind of like this but a bit bigger:
Keep Your Plants All Winter with a Cold Frame | DoItYourself.com
Two hints: replace the glass with fabric or plastic else you'll tend to cook the plants inside plus the glass is really heavy and get an automatic vent like this one:
Automatic Vent Opener
I used this special reinforced plastic fabric in the window because my cats liked to lay across them to sun themselves.
I can't remember where I got it from but it's the stuff you see in the picture here:
Lawn and Garden Greenhouse Kit
Cold frames are great for starting plants - I'd start my cold weather plants in January and cycle through the rest of my plantings as the weather warmed and the plants were put in the garden. It's also great for hardening off plants you raise in a greenhouse like I did or you buy from a nursery before you put them in their permanent home in the garden.
You can also make a cold frame from an old window and some bales of straw, which I've done many times before the permanent cold frame was built:
Gardening : Other : Cold Frames : Home & Garden Television
Good luck and happy gardening!