There are two types of herbicides used in lawns. Pre-emergents prevent weed seeds from germinating, but are not effective on established weeds.
Post-emergents kill existing weeds. Here are some organic options for controlling turf weeds. See this WSU article for more details.
Post-emergent herbicides kill established weeds already present in your garden. Here are examples of organic post-emergent herbicides:
citric acid-- Damage is limited to the area of tissue contact
Perfectly Natural Weed & Grass Killer Organic Herbicide
ammoniated salts of fatty acids--Contact herbicide, will not move through soil.
Concern Fast-Acting Weed Killer
Garden Safe Brand Weed & Grass Killer
potassium salts of fatty acids-- Damage is limited to the area of tissue contact
Safer Fast Acting Weed & Grass Killer Organic Herbicide
d-Limonene (citrus oil)--Damage is limited to the area of tissue contact
Lilly Miller Worry Free Weed & Grass Killer
Pre-emergent herbicides control weeds by preventing any seeds from germinating. Care must be taken not to plant your vegetable seeds when the pre-emergent is still active, usually 4-6 weeks.
Corn Gluten --An organic pre-emergent. It inhibits root growth of susceptible germinating seeds. It is not effective on established perennial weeds.
Bradfield Organics Luscious Lawn Granulated
Corn Gluten, Concern Weed Prevention Plus
Down to Earth Corn Weed Blocker,
Concern Weed Prevention Plus,
St Gabriel PreMerge,
Preen Vegetable Garden Weed Preventer