The following pearl by Jacobson can under no circumstances be left out from the list:
Let $\mathbf{R}$ be a ring with center $\mathrm{Z}$. Let us suppose that you can find $n \in \mathbb{N}_{>1}$ such that $x^{n}-x \in \mathrm{Z}$ for every $x \in \mathbf{R}$. Then $\mathbf{R}$ is a commutative ring.
A good place to learn more about results of this kind is Herstein's Noncomutative Noncommutative rings.

