pyRevit
Short answer
An open-source framework that runs Python scripts inside Revit and exposes a ribbon for community-built tools.
pyRevit is the dominant open-source extensibility framework for Revit. It runs IronPython (and CPython via the bundled CPython engine) inside Revit, gives you a pyRevit ribbon tab full of community tools, and provides a low-friction way to write your own Python scripts that have full access to the Revit API. Many studios standardize on pyRevit because it removes the C#/.addin compile-and-deploy cycle for everyday automation.