Paredit 25 released, after 8 years
Paredit changed the way I write Common Lisp, and I personally consider it a sort of killer feature of developer experience. Combined with canonical indentation of Lisp code (where every line can be indented in essentially just a single correct way, determined automatically by the editor), editing Lisp code actually becomes one of the most pleasurable of activities in programming. When you go back to editing Python, JavaScript, or C—even with a slick IDE—it just feels like a DX regression.