Hi Bean,
Your plant is a begonia.* I'm not sure which one, but I suspect it's a rhizomatous begonia.* You can read the descriptions of the different types here.
http://members.aol.com/btbegonia/types.htm
I found your picture to be very small so I put it on my desktop and enlarged it.* For a better id you might want to ask at this board for the American Begonia Society.* Try and enlarge the picture if you can.* Post it in the 'General Growing Discussions.
http://www.begonias.org/forums/default.asp
I'd love to know what you find out.
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.