NotionX is a browser extension that enhances the user experience of the Notion web application. It offers various features including a dynamic table of contents sidebar, page content statistics, setting the code block language, opening current pages in the Notion desktop app, generating header block indexes, and more. The extension can be enabled by installing it and accessing any address hosted on 'notion.so'. This free and open-source extension aims to improve the Notion experience and welcomes user suggestions for additional features. Visit the NotionX homepage or GitHub repository for more information.