Sweet Alyssum is a great plant for filling in bare spots in the garden.  I plant it among many plants, especially my Butterfly Weed plants (Asclepias tuberosa).  It only grows about 6" tall, blooms from May to December, and is a reliable re-seeder.  It looks magnificent in the fall garden after everything else has petered out.  During hot weather, Sweet Alyssum gives off the scent of fresh-mown hay.  A very small percentage of the plants will get blue flowers on them, but for some strange reason, never until November.