Ajugas are perennials that are commonly known as bugles or bugle weeds, and there are about 40 species in the genus.
Ajuga reptans (reptans means creeping), is often used as a ground cover that carpets the ground with glossy rosettes of spoon-shaped leaves and spikes of violet-blue flowers in late spring.
Cultivars with variegated foliage are less vigorous than those with green or bronze leaves it seems. It is hardy zones 3-9 and prefers light shade, though it will grow in sun too.
In evenly moist well drained soil, Ajuga reptans spreads vigorously, so choose a site carefully.
Cicely Mary Barker, wrote a poem about this plant and called it, as you might guess, "The Song of the Bugle Fairy." It appears in The Complete Book of the Flower Fairies (Penguin Books).