I have a small 6x8 foot greenhouse that I use for starting bedding plants from seed.  I built it in my garage during late winter 2004 and installed it in my garden in the early spring.  The greenhouse was purchased from Harbor Freight as a box of parts that you assemble yourself.  This greenhouse frame is very strong but there are some weaknesses.  The door does not work well, the instructions for putting it together are very poor, it is too short to stand up in if you are more than 5 foot tall, the clips that attach the polycarbonate panels don’t work well, and after 7 years the polycarbonate panels are starting to fail.  That said, it is the least expensive greenhouse on the market and knowing what I know now I would buy one again.   If you are considering buying one of these greenhouses I recommend that you spend some time reading the Greenhouses and Garden Structures Forum on GardenWeb.  If you search for HFGH for Harbor Freight Greenhouse there is a wealth of information about this greenhouse and the 10x12 foot model they also sell.  There are postings that clearly describe how to assemble the greenhouse.  These are invaluable because the instructions that come with it are almost worthless.   The winter after building my greenhouse I posted a summary of how I built and modified my greenhouse to the GardenWeb forum.  It is available here.  I usually start heating my greenhouse and planting my early vegetable seeds the first weekend in February.  If you ready my garden blog you will discover how I grow seedlings in my greenhouse.