I am actually copying this from here, for my own reference/memory:

http://fullofgreatideas.blogspot.co.uk/2011/07/natural-weed-killer-made-with-basic.html - if I'm breaking any copyright rules, just let me know.

I got this frugal, yet extremely effective, way of killing weeds from my neighbor. He was out spraying his weeds a couple of weeks ago and I had to inquire what he was using because all I could smell was vinegar. I love that it is safe for my family and the environment. I also love the price!

I have googled this recipe and there are a million and one different variations so I thought I would share what I used and show you how well it worked.

Materials:

• spray bottle (I had a 34 ounce one I purchased at the $ store a while ago)
• Pickling vinegar (It works better than regular vinegar because the acetic acid % is higher)
• Salt (1/2 cup for m size bottle)
• Dish soap (a squeeze)

That is it!!

Method:

Fill a spray bottle almost full with Pickling vinegar. Since my spray bottle wasn't huge, I added only 1/2 cups of salt. If you spray bottle is big, add more salt. Squirt in a dash of dish soap into the bottle. Close the bottle tight and shake, shake, shake. You are done!!!

Go outside on a bright sunny day and spray those weeds. I found the sun sped up the process so try to pick a good day to do it. Remember though, this will also kill your grass and plants too. So don't get trigger happy like me and kill other things around the garden :)

Here are the before images:

Here are the images after 1 hour (see how the color is out of the plants and they are starting to wilt):

Here are the after/end result images (I just now use a wire brush to get the dried up weeds out of the cracks):

Just to show you how well it works, here is an image where there is a weed still alive and the rest killed. I would like to say I did this on purpose to show you all how well it works but I actually missed this weed :)

The grand total was $0 since I had all the materials on hand. What more could you ask for..costs nothing, not bad for the environment and it gets rid of pesky weeds.

Happy spraying.