While there have been many attempts at a set of such pronouns and none ever became standard, this is the set I see by far most commonly. Several non-gender-binary-identifying people I know use ze/zir/zirs as their preferred pronouns. They definitely crop up in many more places than just here and SlateStarCodex, as someone else replied, but it tends to be mostly in communities that have a particular focus on gender identity.
While there have been many attempts at a set of such pronouns and none ever became standard, this is the set I see by far most commonly. Several non-gender-binary-identifying people I know use ze/zir/zirs as their preferred pronouns. They definitely crop up in many more places than just here and SlateStarCodex, as someone else replied, but it tends to be mostly in communities that have a particular focus on gender identity.