This is something I planted, and I was thinking maybe marigold because that's where I put those seeds. However, I've never seen a marigold that looked like this:

It looks like some kind of spurge. Are the stems hairy? Can't tell from the photos. If so, could be Hairy Spurge. There are a number of spurge species.

It kind of reminds me of arugula or some other "salad green" that has bolted. Not at all sure about that though. Just a thought.

It's not arugula, because I have some of that. Unless it is some kind i'm unaware of.....

I was at a park the other day, and they had little rectangular areas of different types of flowers and shrubs, and this was in one of them as well like it had been planted purposefully

Oh and also they aren't hairy.

Hello,
Looks like an amaranth (red root pig weed) which has had its center stalk removed and has grown lateral branches. Quick way to id the plant is to pull it out roots and all. It will have a central tap root with branching side roots. The portion of the root just below the surface will be red in color with the rest of the root being white. The seed heads displayed in your photo will have small spikes that are sharp when handled with bare hands. Reference link: http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/REDROOT/habit_3_750.jpg

There are some trailing amaranths, and this looks like one of those to me...

I agree it looks like a trailing amaranth, like "Prostrate Pigweed" http://www.illinoiswildflowers.info/weeds/plants/pros_pigweed.htm

I think it looks a lot like that prostrate pigweed, but it might not have such shiny leaves. I guess I'll have to wait until it starts growing again and check it out a littler more closely

Looks like Amaranthus to me. Not sure about the type...also seems a little early in the year.