Agent Orange

... was the most widely used of the so–called 'rainbow herbicides'. Others were agents green, pink, purple, blue and white. You could possibly argue that any of these should be acceptable as answers to this question; but agent orange is the one that everybody's heard of, and it's debatable whether any of the others were "widely used".

The chemical name of agent orange is 2,4–dichloro–phen–oxy–acetic acid – or 2,4–D for short. At least four versions of it were used at different times by US forces in Vietnam. It was previously used by Commonwealth forces during the Malayan Emergency (1948–60), but known by a different name.