Many feeders are designed to prevent bigger-beaked birds from getting at the nectar, but a downy has a fairly long and slender beak, similar to the hummingbird's. I've never seen a downy on a hummingbird feeder, but their presence makes sense. Many birds like sweet tree sap and, in fact, will visit the holes drilled by the Yellow-bellied Sapsucker, a kind of woodpecker whose borings cause sap to ooze from trees. To a Downy Woodpecker, the opening in a hummingbird feeder may be like the sap-leaking hole in a tree. It might have been ants or other insects drawn to the nectar.
Insects are the downy's chief food.
I have seen Purple Finches and Chickadees on a hummingbird feeder. Of course, downies won't harm anything and obviously aren't bothering the Hansen hummers, so their appearance at the feeder may be almost as entertaining as the hummingbirds'.