Bird poop...undigested seeds. Like Curly I sometimes get some weeds but then there have been times like this year where I got an unexpected surprise...a clump of black eyed susans that I didn't plant. :)
Just last week I was watching as a blue jay was pecking away at the spent flowers, and the little finches love them too. Here are some other flowers that the birds love for their seeds:
http://birding.about.com/od/attracti...-For-Birds.htm