At first glance it does look like sawdust (as from cutting wood with a saw) but looking more closely the pieces are not 'dusty' enough. The individual pieces are very uniform, so maybe something left by carpenter bees or carpenter ants or other such wood chewing/dwelling critters...but the color and size are so uniform it looks more like some product that was applied such as Preen (as was suggested) or possibly some other type of weed preventer or fertilizer or possibly insecticide granules? Do you have a landscape maintenance company working in your yard?
Edit to add 'insecticide'.
