I'm finding Erythronium 'Pagoda' a reliable bulb for semi-shady woodland planting. It's an American relative of the European Dog's Tooth Violet (E. dens-canis), said to be a hybrid E. tuolumnense x californicum. 

 I saw something very similar dotted all over a damp meadow near Crater Lake, Oregon a few years ago though that would have been a straight species wildflower. A couple of dozen species are to be found in the States which is most of the genus.

 Actually I never really gave much thought to why it's called 'Pagoda' but looking at these blooms there is indeed a pagoda-like quality.