While looking for a link to show you how to make a green house out of a 2 gallon milk jug I came across this site with 35 uses for the humble milk jug. Quite cool. Viva recycling!
This site shows how to use water filled jugs as "bricks" to make a type of heat regulated greenhouse.
also, Toppers instructions for making milk jugs into planters.
BHG also has a page on 25 Low Cost ( or no cost) Gardening Tools