Periwinkle / vinca minor ground cover. This ground cover gives a slightly wild woodland look — it’s not easy to rake completely clean, so not a good option for the neat gardener. It blooms for a long time in spring, and often again in fall. My parents had it under trees at their house in Connecticut, and I loved discovering them as a kid; I have it under trees here.
This is another one where I won’t be adding more, though, as it’s a sporadic invader of understories in Illinois woodlands, crowding out natives. Alas.