Unity provides hooks into the Project and Hierarchy windows OnGui event. This allows you to add additional flair to these windows. To learn how to enhance these windows see the fallowing documentation pages
- HierarchyWindowChanged
- HierarchyWindowItemOnGUI
- ProjectWindowItemOnGUI
- ProjectWindowChanged
If you want an example project to work off of and learn from there is an open source project called “UnityEditorEnhancements” that you can download as a zip file or you can fork the Mercurial repository.