Internally, we use variants of our pipeline to index a variety of open source repos, and some non-blaze/bazel internal repos. Those are often non-Google repos. But we're using some internal postprocessing and serving logic to actually create and host the final index.

Unfortunately I don't know if there's any significant use of Kythe outside of Google. We get a handful of questions on the open source repo from time to time, but that's all I know about.