The OTW runs Archive Of Our Own, or AO3, a strictly non-profit hosting site for fan fiction and fan art. This is what I know them best for, but they also engage in legal advocacy (defending fan art against exploitation or being sued) and run an academic journal that promotes fandom. And they do it all through 100% volunteer work and the tiniest budget.
As someone who shamelessly fills Kevin’s ereader with Harry Potter fan fiction, I love the OTW’s dedication to provide a safe space for fanworks. If you’ve ever enjoyed a fan fiction (including 50 Shades of Grey and Dante’s Inferno) then this is a charity you should definitely throw your weight behind.