Epimediums, solomon's seal, tiarella.

You are so lucky in zone 6 (oh, the things I would plant if my winters were a little warmer!).

If the shade is cast by a tree, you can have spring bulbs in your bed (with the foliage later covered up by other plants). If your shade doesn't vary by the season, that may not work so well (although daffodils may be worth a try)

Brunnera macrophylla (Siberian Bugloss) has bright blue forget-me-not type flowers in spring.
Toad lily (Tricyrtis) has spotted purplish flowers in the fall, when not much else blooms in a shade garden
Astilbe (takes a year or two to settle in) plume like flowers in a variety of colors and heights. Nice fern-y foliage the rest of the time.

Maybe try a groundcover to fill in between instead of mulch? Ajuga has many varieties with interesting foliage. And Sweet Woodruff (Galium) is a standby for me.
I like wild ginger (Asarum) for the round leaves with a little sheen that play off the hosta. Starts off as a clump but can become good groundcover as it grows.

If you're thinking about plants for the back of your bed with some height & presence, there's Goat's Beard (Aruncus dioicus) that has feathery blooms; Bugbane (Cimicifuga or Actaea racemosa) has white spires of flowers and there are varieties with lovely burgundy colored leaves that are really striking; or Monkshood (Aconitum) which is a poisonous (!) plant that nevertheless has gorgeous late summer spires of blue (there are pink varieties, but why?).