Remember the Louisiana Purchase? You know, the reason America has New Orleans? (Score.)
Thomas Jefferson gets the credit, but Robert Livingston was the guy in France negotiating the terms. He also swore in George Washington as the first President of the United States, and was the final member of the Committee of Five drafting the Declaration of Independence. People may not remember him much now, but if we were in the late 18th century you would definitely know his name.
The Livingston family was a prominent one in New York City, whose members were politically involved and influential on issues like the funding of King's College (Columbia University) and the conquest of "New France" through the French and Indian War. After finishing his own education, young Robert (like a number of these men, he started college at age fifteen, just in case you felt over-accomplished), became a leading figure in the Stamp Act Congress and helped keep the peace in New York in the aftermath of the legislation.
After becoming a lawyer and establishing his own reputation, Livingston became a member of the New York provincial congress, and was then appointed as a delegate to the Second Continental Congress in 1775. There he served on numerous committees, especially ones focused on foreign affairs, as well as financial and judicial issues.
One of those committees was the Committee of Five drafting the Declaration of Independence. However, he returned to the Provincial Congress before the document was signed, so although he helped write the Declaration, he isn't one of the signers.
Once America was independent, Robert really rose to prominence, serving as Chancellor of New York (the highest judicial position possible) from 1777 to 1801. As Chancellor he swore George Washington into office as the first President of the United States in 1789.
Of course, that's not all he did.
From 1781-1783, he the first Secretary for the Department of Foreign Affairs (we now call it Secretary of State), and established many precedents for how that position and department function. Later, he drifted towards the anti-Federalist party of Thomas Jefferson, and was chosen by Jefferson as his minister to France from 1801 to 1803. There, Livingston was the man on the ground negotiating the terms of the Louisiana Purchase with Napoleon.
Like so many other Founding Fathers, Livingston spent the later part of his life involved with a passion project having nothing to do with politics. While in France, he befriended the inventor Robert Fulton, and supported Fulton's successful experiments with steam engines, which led to the invention of the steamboat and brought Livingston a monopoly on steam vessels in New York. The first functional steam vessel was named Clermont, after the Livingston family's New York estate.
Robert Livingston wasn't a famous speaker or author like some of the other men who involved with the Declaration of Independence, which is perhaps why he is easier to forget. But in several ways he had a significant influence on American government and industry. He was known even in his own time as "Cato" or "The Chancellor," and you have to be pretty legit to earn classy intellectual nicknames like that.