Answer: There are so many to choose from! If possible, visit a local nursery so you can see some possibilities "in person".
Spring bulbs will do well since they will bloom before the leaves come out. Ferns are great as well, especially if the site is moist. Here are some other possibilities:
Anemone (windflower), Geranium spp. (cranesbill), Epimedium (bishop's hat), polemonium (Jacob's hat), Viola, Tradescantia (spiderwort), Digitalis grandiflora (perennial foxglove), Astilbe, Hosta, Tiarella (foamflower).
Also, if you look through Burpee's catalog you'll see a symbol that indicates whether a plant will thrive in shade.
Hope this helps you get started!
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