I call mine volunteers too. Especially now as I am slowly turning my flower gardens into low maintainence shrub and perennial borders.
The foxgloves and lupines are two of my best volunteers along with the spirea. Just hate pulling out and discarding spirea bushes when I paid good money for the parents. But man they pop up everywhere!!! I know the answer is not to let them set seed, but I just can't seem to keep up with that now, with my physical problems and all.