Someone here (I think probably MarcP) identified it for me a couple years ago -- mulberry weed.
I didn't recognize it at the time, because it was relatively new in my area.
It is very nasty aggressive annual weed that spreads like crazy. Once those seeds let go, then thousands of them pop up. Ever since I got it ID'd, I pull it every chance I get - the other day, I was over at my next door neighbor's pulling them from her yard, or otherwise hers will spread back to my yard.
It can grow anywhere, including in pretty much total darkness, so it pops up under the leaves of other things. So I have to go around lifting up the leaves of all the hostas and prostrate juniper and etc, pull the mulberry weed out from under.
"This devilish weed is native to Asia and was first recorded in the US in Louisiana, in 1962. Since then it has spread throughout the southeastern United States in nursery material, and is present in every state east of the Mississippi, from Florida to New England. Some states west of the Mississippi are beginning to report Mulberry Weed in nursery stock, greenhouses and gardens as well. As weeds go, this exotic invader is a relative newcomer, but it is a persistent problem in southern gardens and a serious threat to nursery stock. According to one study, by 1997, over 50% of the southeastern nurseries inspected had this weed growing in their stock, and by 1998, the number had climbed to 75%. Thatâ€™s how fast this weed is moving."
there's more information about it in the above article. It is pretty easy to hand pull, except you want to be careful not to break the stem and leave the root in the ground. I wouldn't recommend using poisons on it, because it is one of those invasives that easily develops resistances. It is already resistant to Preen, which will not harm it.
Bottom line: get this one out of your garden ASAP, before it goes to seed!