I agree with using hostas. We just put a hosta bed under a live oak in our back yard--it gets morning sun, so keeps the color true on the hostas. And the deal about protecting the trunk of your tree is for real!
Perennials: For ground covers, look into sedums. There is a really pretty bright, yellowish-green one with flat, circular leaves that brightens shady areas. I don't have it anymore (I planted it in the sun--duh!) But I have another kind under my Magnolia tree that is light green, and spreads really well. I'll see if I can take a picture of it, 'cause I don't know the name. Also, wandering jews (purple heart and fuzzy) actually came back after our especially cold winter.
Annuals: Try houseplants in that space. I get tired of begonias and impatiens, so I've put crotons, asparagus and boston fern (which will probably die out when it freezes), and pothos ivy in my third shade bed.