Glyphosate, diquat and 2,4-D are a few common chemicals found in weed killer, according to the Environmental Protection Agency. About.com mentions additional chemicals such as MCPP and dicamba. Atrazine is another chemical found in weed killer, according to Wikipedia.org, along with TCDD and Triazine. For post-emergent weed control, chemicals such as fenoxyrop ethyl, dithiopyr and quinclorac are also used.
According to Pennsylvania State University, carfentrazone and clopyralid are two other weed chemicals. Other chemicals such as MCPA, triclopyr and isoxaben are also used. Pre-emergence weed killers are usually applied during the summer to prevent weed growth. Chemicals in this category include benefin and trifluralin when used in unison. Benefin itself is another effective weed killer, along with bensulide, oxadiazon and siduron. Prodiamine and pendimethalin are more chemicals. Agent Orange is another herbicide that kills weeds and other plants, according to Garden Guides.
Wikipedia.org notes organic-based chemicals in weed killer, including corn gluten meal, which is used in pre-emergent weed control. Vinegar is another effective method of killing weeds because of its acetic acid content. Citrus oil strips the cuticle away from weeds, causing dehydration in the plant. Saltwater applied at the root level of weeds is another way of achieving weed control. Garden Guides mentions alcohol as another element commonly found in natural weed killers.