For spring, try some Phlox stolonifera. Like a groundcover, yet beautiful shades of lavender grace long stems. But it sounds as if you have ground covers, um, covered. Good for you having Trilliums!!! How about some Hellebores for dramatic foliage and super early flowers? Thalictrum with Hosta is lovely. Some tall white Nicotiana (annual, of course, but does self seed) would be nice in amongst the ferns. Clumps of Galanthus in the Galium would be a nice combo. I could go on.... Good luck!

Try GardenHere.com....Very nice very HELPFUL people there... Good luck with your garden...
Grace

Here on Garden Web there is the Companion Plants forum. I haven't been there much and can't say what kind of advice is usually available there. You can also try Gardening in Shade (which with those plants is what you should be doing!).

Here is a link that might be useful: Companion plants forum

Just Google woodland plants, gardens etc, you will get lots of ideas.

There are many cool sedges that love shade that look nice with hostas, one I love is Plantain leaved Sedge, Carex plantaginea. It is 8" tall and kinda looks like a mini Hosta. I also love Hakone Grass Hakonechloa macra, It lends a oriental feel 12-18".
Other stuff that hasnÂt been already mentioned on this thread I think should be in every shade garden:
Various Columbine species
Bleeding Hearts- Grandma plant
Celandine Poppy, Stylophorum diphyllum- good self seeder
Foam Flower, Tiarella cordifolia- Good mixer, nice fall leaf color.
Wild Ginger, Asarum canadense- opalescent sheened, kidney-shaped leaves
Bluestemmed Goldenrod, Solidago caesia- Adds needed fall color, but can over seed
SolomonÂs Seal, Polygonatum biflorum - Strong structure & texture
Spikenard, Aralia racemosa- takes role as shrub.
GoatÂs Beard, Aruncus dioicus- also shrublike if you have the room.
You are not going to get a whole lot of bloom color in the woodland, especially after spring. ItÂs mostly texture that is the most important thing. You can also plant a few annuals here in there too.