I started off with a cold frame, then added one of those zip up plastic 4 shelved jobs. About 12 years ago, I got an 8x6 one from Hombase, I think. I put it up with a neighbour, for which I was very grateful as I had a nasty accident with glass a few years back and I'm neurotic about the stuff. Since then, I've added a 4x2 lean to.
Bloke electrified them both a couple of years ago, which makes a huge difference, now I've got a couple of heated propagators and a little greenhouse heater in there as well.
I also got a small water butt and hooked it up to the guttering on the bigger one, so there's water on tap all the time.
I do love them, I've overwintered my brugmansias in there, and they've still got all their leaves. I am lucky enough to have a walled garden though, and the Walls seem to act as giant storage heaters...I haven't bothered to bubble wrap them for years now. ( the greenhouses, not the Walls!)