Many of my favorite plants could never be the "star of the show" in a garden design, but enhance the beauty of neighboring plants and the garden as a whole. I like a full, lush look, and the use of these "shoes and socks" plants really fill out a planting.
Various hardy Geraniums are among my favorites, they happily fill in spaces between other plants and provide great contrast for many prominent forms in the garden, especially plants with sword like foliage.
What are your favorites?