Please excuse the somewhat stark photography. I'm currently in tech week for the show I'm in, so I don't get home until well after dark. This plant is growing in the patch of land where the grass is still failing to grow even after adding more soil and reseeding with a shade/drought-tolerant species.
As can be seen in the pictures, it's got a 7-pointed lobed leaf. My first thought is that it looks a little like a tree, maybe a poplar or maple, but I'm not botanist. I'm happy enough that anything is growing there, but I'm curious. If it helps, all of this sprung up within a few weeks.
Here is a close-up of one of the leaves: