If you have an unused or gently used pair of eclipse glasses you'd like to donate, here's what you can do. Visit Astronomers Without Borders' homepage for the eclipse glasses donation program. They've partnered with libraries, museums, schools, and eyecare centers across the country, which will serve as collection points for donations.
As you'd expect, many of the collection points are focused around or near large cities. If there isn't one near you, you can spread the word to local organizations that may be interested in serving as collection points -- there's an easy sign-up form on AWB's site to do just that. Once glasses have been gathered (the organization expects to run the drive through at least the end of September), collection points can mail them in to Explore Scientific, which is responsible for housing the glasses until they are redistributed.
And if you're concerned about the safety of used glasses -- after all, there were dire warnings about using fake or damaged eclipse glasses -- fear not. Astronomers Without Borders will be checking each pair of glasses to ensure they are safe before distributing them. It's a chance to do a little bit of good, rather than just throwing your glasses out.