These are pretty much always called Hendricksons, Sulphurs, PMDs, BWOs.
The Ephemerellidae are undoubtedly the most important family of mayflies for the American trout angler. Most species known as Hendricksons, Pale Morning Duns and Sulphurs belong to the genus Ephemerella in this family, while the genus Drunella lays claim to many Eastern and Western Blue-Winged Olives and the Western Green Drakes.
This is the most widespread species of Ephemerella, and also the most abundant in some places, but nobody I've talked to seemed to know what its duns looked like, and there were no pictures of its duns online or in any angling books. That mystery is solved with this male dun, which hatched from a definitively identified nymph.
This spinner molted from a dun after being photographed, and the dun form is listed here as a separate specimen. I've rarely found a more cooperative and photogenic mayfly.