www.harborfreight.com has a pair of aluminum-frame greenhouses.
http://search.harborfreight.com/cpisearch/web/search.do?keyword=greenhouseThe 6x8 has a good track record, and the 10x12 has a mixed record. I'll explain:
We bought ours early last year and discovered that this was just after the product was announced. Manufactured in China, the instructions were clearly never checked by someone who actually tried to assemble the unit with them and no prior experience. It was simply a nightmare.
A bunch of us gathered around this forum and exchanged notes until a handful of the units had been assembled and customized.
http://forums2.gardenweb.com/forums/strucs/By now, a year later, the 10x12 is probably a safe bet. The instructions have been refined and my recent visits to the forum reveal far fewer folks threatning acts of violence during the assembly process. Clearly you will receive as much value for the purchase in the poly panels alone.
If you get this unit, be sure to look over the various posts. Many good ideas about VERY NECESSARY reinforcements have been posted. If you have the HFGH in a windy area, or where you get snow, it needs to be reinforced. $100-$150 of materials will make this unit very strong.
Ours is mounted on 2x12 lumber so as to provide a rigid footing, we added angle aluminum over the doorway, and rigid conduit to tie the side walls to each other. Then we added raised bed boxes inside, steps from the outside, and are really very happy with the unit. Veggie seeds we planted a month ago, despite snow and freezing temps, have sprouted 1/4 inch and are growing steadily.