Natural (and Cheap) Weed Killer

Kill off your weeds using the old time recipe of Vinegar, Salt and Soap.
1 Gallon Vinegar (draws the moisture from the plant)
1 Cup Salt (kills off the plant)
1 Table Spoon of Liquid Dish Soap (breaks surface tension on the plant)

Note 1: This recipe will effect all ground cover (i.e. grass and flowers) as it makes the the ground more acidic and draws the moisture from the plant. Thus, it is NOT killing the plant, just removing its moisture... which is enough to kill off some plants though not all.
Note 2: As the vinegar does not penetrate the plant down to the roots, over spraying the plant (so the vinegar seeps into the ground) may be necessary. Also, as with many natural sprays, reapplication will be necessary to 'kill' off the mature (and stronger) weeds.
Note 3: The soap helps break the surface tension on the plants leaves. This then allows the vinegar to penetrate the plant.
Note 4: Salt stays in your soil. It also makes it harder for plants to grow back. So... use caution as to where you spray.