I'm having trouble figuring out what plant I have here. By flowering head I thought some sort of milkwort, but the leaf structure isn't like the cross leaves milkwort. I'm in west central Illinois and it is in a prairie habitat with goldenrod, indigo, ironweed, and bergamot as some of the more common flowers. Its fried up a bit in the picture.

It reminds me of some sort of Trifolium

Maybe Justicia. Trifolium has trifoliate leaves.

Looks like seed head of Prunella vulgaris.

It did remind me of Justicia but I didn't even pay attention to the leaves; I think Christine has nailed it with the ID of Prunella.
Self-heal (Prunella vulgaris)
Self-Heal (Prunella vulgaris subsp. lanceolata)