My daughter came home from school one day with a pop bottle that had been cut in half and had some dirt in it. It now looks like this:
You take a pop bottle (20 oz. or larger) and cut it in half. Take the cap off and puncture a hole in the middle . Tie a knot in a piece of yarn and feed it through the hole. Tie another knot on the outside, so that there is a knot on both sides of the cap and extra yarn on both ends. Screw the cap back on and fill the top half of bottle with potting soil. Plant your seeds according to their packet. These are marigolds, but you could do anything. Fill the bottom of the bottle with water, but keep the water level below the knot. Take a spray bottle and wet the dirt pretty thoroughly on top just on the first day. After that you should only have to water it from the bottom. When the plants get bigger you can transplant them outside or to a bigger pot depending on what flower you've grown. If you leave it in the bottle the roots will eventually become too much for it and it'll become pot-bound or bottle-bound in this case.
I'm thinking of planting all my starters for my garden this way next year.