Redmine inclines to using dummy projects as project categories. This always seemed odd to me, as in this case we get false projects, mess of ones, that play roles of categories, and the real parent projects.
This plugin is a try to implement the real project categories for Redmine, which it calls “sections”. The primary goals of sections are: to categorize projects, group projects in the project lists and additionally describe projects (e.g., Redmine › Project Sections). The plugin also implements it in an unusual way: thus, sections are used to build project URLs, sections support subsections, subsection with the same name (and identifier) can exist under different parent sections and so on.