I originally got this idea from Money Saving Mom. It is working pretty well and I hope to have several tomatoes and peppers to plant when the nights finally warm up enough to be planted!
I started out by cutting several milk jugs horizontally into two pieces. I poked holes in the bottom of the milk carton so excess water won't accumulate. Then I filled with dirt and planted one seed per milk jug. I labeled my jug on the top section with a sharpie. I then used packing tape or duct tape to completely tape the two sections back together - throw the lids away or find another use for them. I left my jugs like this several weeks. It was cold at night and during the day but the sun warms up the air and within a week I had little tiny tomato seedlings growing!
We have had an unusually warm spring this year, so I had to "open" my greenhouses earlier than expected. To open then I cut the tape on 3 sides and left one side attached. I can open them during the day and close them in the afternoon. I like to close them when they will still get some time in the sun so the air temp can build up in the jug before the sun goes down. In the mornings I can open to water as needed.