An Amaranthus sp.

Here is a link that might be useful: Amaranth

amaranth, a potherb.

Mine is grown near tomatoes too!
I harvest the top, and cook it (leaves and stems) for 2-3 minutes with a little oil, garlic and salt. It tastes like spinach. The bottom will soon grow tall and bushy, so you can harvest every few days. IMO, certainly a great plant to have in a summer garden - pretty, no fuss, productive, and good tasting.