I've had good luck with plants.usda.gov identifying plants by geographical location. Just use the advanced search, choose the location you found the plant, e.g

I've had good luck with plants.usda.gov identifying plants by geographical location. Just use the advanced search, choose the location you found the plant, e.g. state:county, and as much information as you can deduce, flower color, growth form, etc. It'll usually provide pictures and interesting things like whether Native Americans used it medicinally, etc.