(Revised 17 December 2013)
I have not found too many attractive flowering plants for shady part of my garden. Acanthus has lovely upright spikes of white and dark purplish flowers, interesting large foliage, and seems to do fine in a fair bit of shade.
Some books mentions that it can take very bad soil and hard to remove. So far, it has not spread in any significant way. May be the cold weather here keeps it in check. My plants came from seeds I sowed quite a few years ago. The hardiness rating of 6-7 would mean that it is borderline hardy to my garden. So far there is no evidence of that.