Century Plant {Agave americana};  wide, flat & parallel to the ground, succulent, sword-shaped, greenish-blue leaves growing in a rosette pattern.  A long caudex (stalk) often grows straight up from the center

Southwest High Desert Yucca {Hesperoyucca whipplei}

Bibliography: http://en.wikipedia.org/wiki/Agavaceae

Database Entry: Distance Everheart 5-8-13, 5-14-13, 12-28-13