Trade associations (501(c)(6) in the US; Eclipse and Linux Foundations are examples) are another category that may or may not fit your schema, roughly nonprofit

Trade associations (501(c)(6) in the US; Eclipse and Linux Foundations are examples) are another category that may or may not fit your schema, roughly nonprofits governed by (usually) for profit stakeholders. plos.org archive.org and eff.org are additional direct nonprofit (charities, 501(c)(3) in the US) examples. EFF is an advocacy org but they do more interesting tech work than the vast majority of startups. Another interesting category maybe worth considering are for profits with normal for profit governance but some non-fuzzy public benefit aspect at the core of their business. In the context of tech a free-software-only for-profit like Red Hat maybe. You probably want to put a finer point on "social impact".