Try astilbe. It blooms nicely in shade and comes in a lovely pink or white feather-like blossom.

Campanula persicifolia has the longest bloom time for me. Telham Beauty is a blue beauty. Heuchera's also have a long bloom time, but the foliage is what adds the interest.

I LIKE THE BURGUNDY LACY FERN. NICE RED COLOR YEAR ROUND

i just posted a picture of my new torenia basket in another entry. lovely blue flowers. it's more of a ground cover/ hanging plant and works great in a fully shaded area. it can also take some sun, but not necessary. don't know if that's what you're looking for...

Endless Summer Hydrangea

Lamium is a good choice.
This year the flowers on mine seemed to last a while--several weeks, plus different cultivars bloomed at slightly different times, and with different varieties you can find nice silvery foliage and pink or white flowers for your scheme. They are low-growing, though, so best for the front or spreading beneath other plants.

Lobelia (siphilitica and cardinalis) each last a long time for me - almost three months if I keep it moist.

Dicentra 'Luxuriant' is a long bloomer, my cyclamen is still blooming, hellebores will hold their blooms for several months (Jan-April for me), as for annuals, impatients and wax begonias and torenias will do well.

My perennial forget me nots are beautiful pale blue and bloom for several weeks in the shade.

Pulmonaria has silvery leaves and early flowers, and lamiums also have silvery leaves.

I planted Aruncus dioicus (Goatsbeard) for the first time this year and it has grown to three feet tall very quickly. It has developed the most lovely frothy white plumes which have been blooming for two weeks now and show no sign of 'browning off'. I love this plant!

There is a variety of tradescantia whose name escapes me but the flowers are a light bluey lavender colour....