I am writing this on the spur of the moment. I will add more details or another article every couple of days. But the cheapest and most environmentally friendly weed killer that I keep handy is a jug of vinegar. I put the vinegar in a spray bottle and apply it directly to the weeds at the hottest point in the day, when direct sun is most available. The weeds will dry up and then be easily pulled out of the ground. This works to permanently kill a weed, but does not mean another one wont grow later. If you leave the dead weed it will yellow and it will actually prevent another from growing. But thats a bit unsightly.