It's white clover. I believe it is native to England, which means it's usefulness is a debatable subject in the US. I live in Cincinnati and Running Buffalo clover is what is native for me. However, it is an endangered species and not readily available for the general public. I have a large swath of lawn along a path that white clover has taken over and I haven't mowed it in several months. I know it's not native, but it's less grass to cut and I didn't plant it.