I'm not sure what it is. Knowing my eye, it's probably a weed! But then again, someone probably sells them somewhere, too. It appears to spread by rhizome, or at least by runner. It has tiny flowers. It wasn't much taller than the lawn from which I rescued it today. Any ideas?
Malva rotundifolia (Low mallow) or possibly malva neglecta (Cheeses). They're related to the Hollyhock mallow and the seedlings look exactly like them. I've left a few in the garden thinking they were the zebrina hollyhock. BTW - the mallows are used as medicinal herbs.
Do yourself a favor and get rid of it now before it seeds all over your garden, it really is a weed and has little to recommend. There are plenty of mallows that you can plant that are much more attractive than this.
Gardenwife: Jerseyguy is right, throw it away, you don't want to get those weeds established in your yard or garden. They are perennial and there roots go to china, when you are trying to pull them. Also very invasive, reseeds viciously.