Nephrolepis cordifolia
AKA "Sword Fern." This is an invasive fern, but as invasives go it is pretty mild-mannered in the garden. It does require regular hacking back and digging out as it surrounds and engulfs other plants, but it works remarkably well as a ground cover because of that tendency.
This photo is of a mature stand of Nephrolepis cordifolia on the Cal Poly campus.
And here are my two small cuttings, planted Spring 2006, in the Fern Walk:
Because of the size of the Fern Walk and the general expense of buying ferns, I am hoping these guys fill in and take over their little patch of land.