That is a lovely little thing! It appears to be a double cowslip (Primula veris as Carol says). A British native in its single form. I've never seen a double. Cherish it. By that I don't mean mollycoddle it - it is a native of hay meadows and open downland and is not used to rich living. Don't be tempted to feed it or fiddle with it. It looks happy just where it is.

oops... here's a "clickable" link... Here is a link that might be useful: http://youtu.be/mOhyCo5qjHU

Hard to tell without seeing the whole plant if it isn't V. anagallis-aquatica or another similar species. It isn't V. scutellata though because it typically has flowers on longer pedicels (and very slender leaves if you can show those).

Man that really sucks, but mowing over it will actually make it worse because of the plants ability to take root from single pieces of plants, even a single needle can become a separate plant. A lot of succulents are able to do this quite easily. I still have the stuff growing back, but most of it is growing underneath bushes and other plants and just pull it when I notice it. I got rid of periwinkle the same year, 2 years ago, both have a few plants popping up here and there my next door neighbor planted English ivy, and it is spreading into my yard, I'm going to ask him if I can pull it eventually and offer to plant an alternative that isn't such a destructive plant. English ivy in Michigan isn't usually so bad because of the winters here, but a few warm winters and it could become much more invasive.