Asplenium trichomanes (Maidenhair spleenwort)
 
Don't let the delicate appearance fool you. Maidenhair Spleenwort looks fragile — fine dark stems, small rounded pinnae, a rosette barely bigger than your hand — but it is genuinely tough once established. This is a fern that has been quietly colonizing rock crevices and shaded cliff faces since before your garden existed, and it will outlast most of what you plant around it. 
 
It forms tight clumps about 6–12 inches tall and wide, and in nature grows on shady limestone cliffs and in rock crevices. In the garden, it asks for sharp drainage, some shade, and the courtesy of not being fussed over. Rock gardens, stone walls, containers, and shade beds all work — and it's evergreen, deer resistant, and drought tolerant once settled in. 
 
If you have a shady corner with rocky, well-drained soil that you've been staring at wondering what on earth to put there — this is your answer.
 
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photo by Lazaregagnidze