Hulled Millet is an easy, inexpensive addition for most feeder combinations. Best served "on the ground", Hulled Millet is no-mess in that it should not sprout and will not leave hulls behind.
Millet is wildly popular with most "ground feeders", that is, birds that prefer to forage on a flat surface as compared to feeding by reaching into a feeder-port while perched.
All doves will eat millet readily, as will Juncos, both Spotted and Green-tailed Towhees, all the various sparrows (yes, the native ones, too!).
Finches will eat small amounts of millet if it is mixed with sunflower or Nyjer.
While most "blackbird species" will avoid millet when possible, Red-winged and Yellow-headed Blackbirds will eat millet readily either while perched or from a platform, and in winter may do so in large numbers.
Grackles, Starlings, Jays, Crows, Magpies, and all the various Woodpeckers (including Northern Flicker) should avoid millet as long as alternatives are available (alternatives can be naturally occurring or offered at other feeding stations).
Offer millet in a tray or platform, along a railing, or by sprinkling it on the ground under feeders and along the edges of a fence or hedge. Small amounts of millet can also be added to any mix in any tube or hopper feeder.