inside of a desk compartment that is intended for a tower. it's raised a few inches off the ground. most of the pc setups I've done for personal or family use are on the ground or on a raised platform. I have more desk space this way.

at work, where I do desk side support among other things, usually on floor as well. only put it on the table in cases where cables are simply not long enough or there are no holes for cable management in the desks.