Among the least wonderful properties of the modern web is the way it allows strangers to execute JavaScript code on your computer. While this facility is responsible for the very existence of rich web applications like Google Docs, many more curmudgeonly users (the author included) often wish for a bit more control about what code their own computer executes. uMatrix is a browser extension that provides this control. It allows users to build a per-site whitelist specifying which scripts and plugins can be run, which cookies can be set, which images and media can be loaded, and which other requests to other sites are allowed. The "Walkthrough for first time users" on the uMatrix wiki describes how to build these rules. As the uMatrix github page notes, the plugin is "definitely for advanced users" and most sites will be broken before the user adds the necessary whitelist entries. uMatrix is available for Chrome, Firefox, and Opera.