φ!
So I stumbled upon this useful page: list of strange units of measurement and learned a very interesting fact: One mile is within 0.5% of φ kilometers. The golden ratio is 1.61803399; a mile is 1.609344 kilometers. So if you ever forget how many km to a mile, that's a good way to remember (or conversely if you forget the golden ratio or forget the eigenvalues of [1 1; 1 0], but you remember the number of km in a mile, this could come in handy).
And as the above-linked page points out, similar useful facts, see mathematical coincidence for fun facts like π2=10 to within 1.3%, or that sin(666°) = cos(6·6·6°) = −φ/2, exactly(!)
And as the above-linked page points out, similar useful facts, see mathematical coincidence for fun facts like π2=10 to within 1.3%, or that sin(666°) = cos(6·6·6°) = −φ/2, exactly(!)