"Related Files" for Brackets. This extension scans files when they are opened/added to the working set and builds a list of related files in the project. You can fold down the related files via a discloser to the right of working set files and can open the related files from there.
The list of related files is determined by searching a document for relative paths to each of the files in a project. A few tweaks are in place so that it can find requires in JavaScript.