Glyphosate-resistant Giant Ragweed Over-topping Roundup-Ready Corn. Photo by Dr. Bill Johnson.
Source: International Survey of Herbicide-Resistant Weeds.
The same website has a list of herbicides, ranked by the number of weeds that have evolved resistance to them.
From p. 4 of Darwinian Agriculture:
"Widespread use of transgenic crops resistant to the weed-killing herbicide (see glossary) glyphosate presumably increases the use of that herbicide, while reducing the use of other, more-dangerous herbicides, at least until weeds evolve resistance to glyphosate."