Selaginella Ferns- Easy to Grow, Hard to Find
There are varieties which grow closely along the bark on tree trunks, other varieties that resemble little lace parasols strung out along lines on the ground. Most of the tropical species make groundcovers easily enough. Some of the temperate species look like miniature Spruce or Fir trees a few inches tall.
In almost all species the culture requirements are the same: provide plenty of moisture and "freckled" light meaning that you should not have direct sunlight at any time, just the shifting patterns you'd see under an oak tree. Most of the Rainbow Ferns grow well on acid, organic soil, although many grow on limestone rocks to a brilliant effect. If you take the time to track down some of these ferns, and provide them with the conditions they need, you'll find them easy and rewarding to grow.