KaTeXify
A browser extension that renders LaTeX mathematics on pages that do not, such as GitHub issues and READMEs.
Written for the years when GitHub showed $\sigma$ as four literal characters: KaTeXify scans a page for inline and display math and typesets it with KaTeX in place. It was a small tool with a steady following until GitHub added native math rendering, which is the best thing that can happen to a browser extension.