Ron L. Graham is sadly no longer with us.
He was very prolific and his work spanned many areas of mathematics including graph theory, computational geometry, Ramsey theory, and quasi-randomness. His long association with Paul Erdős is of course very well known. Graham’s number, and Graham-Rothschild theorem, and the wonderful book Concrete Mathematics are other well known contributions.
However, some of his contributions may not be as widely known, but deserve to be so. This question is to encourage people to comment on such contributions. I am not familiar with his work on scheduling theory, for example.
He was into magic tricks and the mathematics behind them and co-authored a book on this with Persi Diaconis. And he was into juggling, like Claude Shannon.
Edit: Thanks to @LSpice for pointing out the Meta MathOverflow thread here on personal anecdotes.
Best Answer
The largest small hexagon determines the largest area a plane hexagon of unit diameter can have. (No, it's not the regular hexagon!) I love the title. For further results in this direction, search for Mossinghoff's work on isodiametric polygons.