Here's a fun thought: As humans dig heavy materials out of the earth and build taller buildings, we increase the moment of inertia of the planet. This theoretically slows the rotation of the earth.

Of course, given the huge mass of the earth, the effect so far would be negligible. But maybe it would be noticed in the distant future if it were done on a much much greater scale than it is today.

As more food for thought, we are also changing the moment of inertia by putting so many planes in the sky, and by sending mass to space. If every plane were to land, the earth should rotate slightly faster.