For those interested in conservation, one can argue the most important piece of legislation enacted was the Federal Aid in Wildlife Restoration Act, more commonly known as the Pittman-Robertson Act. This act was sponsored by Nevada Senator Key Pittman & Virginia Congressman Absalom Willis Robertson and was signed into law in 1937 by Franklin D. Roosevelt. Backed by hunters, it established a tax based system to fund conservation programs through an excise tax on guns, ammunition, and hunting license fees.
Take a look at this infographic showcasing a brief history lesson on the P-RA:
For more information, check out this NSSF's PDF