Looks like common mallow (Malva neglecta). Generally it is a pretty annoying weed.

They have a long, woody taproot that is hard to pull out. If you want to get rid of them, make sure to not let them go to seed.

Here are some images:
http://calphotos.berkeley.edu/cgi/img_query?query_src=photos_index&where-taxon=Malva+neglecta

They are already growing all over the place...:(

Here is a site that deals with removal:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74127.html

You can eat the new leaves of mallow but they aren't all that palatable. If you cook them, they'll get kind of gummy like okra. They also have pretty little flowers.

You could yank some of them, but I like a few mallows in my garden.