It's some form of amaranth. Could be a weed species, but the purple pigmentation suggests that it is possibly one of the cultivated ornamental/edible varieties. Whether weed or domestic, it is edible as raw or cooked green, may have attractive flowers, and would produce edible seeds eventually.

Agree with Deninmi, amaranth of some sort. If you don't want millions next year, either remove it before it sets seeds or collect the seedheads. Cheers!

Thank you both for the insight! I've been weeding this for 2 weeks and they don't seem to be coming back; so they must be valuable. = )

Just potted the last of them, so I'll wait and see if they look like all the Amaranth pictures I'm seeing.