nbviewer is a web application that lets you enter the URL of a Jupyter Notebook file, renders that notebook as a static HTML web page, and gives you a stable link to that page which you can share with others. nbviewer also supports browsing collections of notebooks (e.g., in a GitHub repository) and rendering notebooks in other formats (e.g., slides, scripts).
nbviewer is an open source project under the larger Project Jupyter initiative along with other projects like Jupyter Notebook, JupyterLab, and JupyterHub.