One of my recent posts was all about discounts on admission and mentioned the North American Reciprocal Museum Association, which gives you access to over 800 institutions just by buying a membership at one of them, preferably wherever you can get the best deal.
The Harn Museum at the University of Florida now offers free admission- which includes NARM, ROAM, and CUAM access. THAT MEANS HUNDREDS OF MUSEUMS FOR FREE! Check it out here. Essentially a digital membership card is free, you can support the museum separately, and the reciprocal benefits were provided by a donor.