Violets, but they will spread if you're not careful, periwinkle (vinca) which is also a ground cover ... my whole yard is pretty dark, and some plants, like columbine, which are supposed to be okay, died. Astilbe will do okay in some shade as well, any garden center has "shade tolerant plants" in a separate area. Impatients and begonias, which are annuals, do well in shade as well.