I bet you'll recognize this one if you've walked any of the forested trails in our area. It covers the ground, creeping over rocks and logs, never growing more than about an inch tall. It sports cute little vine-like stems with glossy green leaves, and of course the tiny pink flowers (always in a pair, because, you know, twins).
 
Twinflower is a beautiful groundcover for your shaded and somewhat moist areas. It will slowly creep along and fill in around established plants, but won't out-compete them. It does a nice job filling in beneath denser shrubs.
 
Twinflower likes rich soils and shade, and does not like full sun or extended dry spells, although it can withstand some drought. Good companions include fragile fern, raceme pussytoes, elk sedge, pinegrass, and Oregon grape.