What is h-index? It's a measure of academic productivity.
Is it useful? Well sort of. You can read more about it on Wikipedia.
I wanted to apply this measure to github stars, instead of academic citations. I've called the result the gh-index. Like any metric, it's not perfect, especially because authorship of open source software can be a bit more complicated than authorship of academic papers. But h-index was never a perfect measure to begin with. So take it for what you will. This tool will calculate the gh-index for any github user you'd like.