"Wall of Water" and much cheaper. I can punch holes in them and they will water a plant for quite some time if I place the jug near the roots. They would make great seed starters too - See my post about that here.
I had a big scrap of pretty cellophane left over from a project at work. I wanted to plant seeds I had saved last fall but didn't want to turn on my mist system quite this early. I decided that the cello would be the ideal thing to retain moisture in the seed bed. I re-purposed a few little plastic pots to keep the cello from being in direct contact with the soil, scattered my seeds and misted the bed well. Next I covered the whole thing with the cello sheet and turned on the heat cable under the soil. As you can see the seeds sprouted and will continue to grow here until they are big enough to pot up.