Kuker - Debug applications made with React, Redux, Angular, Vue and many more.
OctoLinker - Lets you navigate through projects on GitHub by making references in code clickable.
Octotree - Gives a file tree view on the side of each repository that you visit that you can use to navigate over the codebase.
Open GitHub in IDE - This extension allows you to open files in your IDE directly from GitHub, assuming the repository you are working on is cloned on your computer. When a fragment of a file is displayed, your IDE opens the file and puts the cursor at the desired line.
Pesticide - Quickly debug CSS layout issues by toggling different colored outlines on every element.