"Tag on X" is a Chrome extension developed by up2date.3. This free tool allows users to add notes to their Twitter friends for easy recall. It simplifies the process of tagging or remarking someone on Twitter, ensuring that users can keep track of important information about their followers even if they change their names.

The main feature of "Tag on X" is the ability to tag someone on Twitter, making it convenient to add and retrieve notes associated with specific followers. This tool enhances user experience by providing a simple solution to organize and remember important details about Twitter connections.