A couple of days ago I used ordinary store-brand vinegar to kill weeds in my driveway. Just sprayed the vinegar directly onto the weeds, and by the next morning the weeds had started wilting.
I found applying vinegar to be much more economical and environmentally friendly than using herbicides. A 2-gallon bottle of 5% white vinegar cost around 2 dollars. Compare that to the price of your favorite (and much-advertised) herbicide.
You can pour the vinegar on the weed straight from the bottle, or load some vinegar in a spray bottle and squirt it on the offender (like I did).
I sprayed the vinegar in the morning, on a day when there was no chance of rain. (Vinegar degrades in water.) I think I'll spritz some weeds in my lawn next -- see what happens.