Milkweed, Spotted Knapweed, Vetch, many other flowering plants. The nectar flow is still going strong and seems that it will keep going a while yet. If the honey supers are filling up, put on two more supers and the bees may fill them up.
Several new beekeepers have stopped in and purchased more supers. They were surprised to get honey their first year. Most beekeepers should be getting four supers of honey or more this season.