I had to think hard about this, because the chances were that my most useful utility would be so useful that I would no longer think about it, if that makes sense.

It has to be Kawecki's Prop Viewer. I have it associated with OBJ and many Poser library files on both computers I use regularly, as well as a portable version to carry around on my USB drive, so I probably use it several times per day on average.

At the risk of lapsing into self-promotion, the content that I use most often is something I made. My Basic Backdrop Box finds its way into nearly every scene I work on, at least during the set-up stages; it can also appear in the final render once appropriate textures have been applied.