The Project Manager is one of the oldest tools built into VFP, and it has showed its age for a long time. For example, it doesn't provide integration with modern distributed version control systems (DVCS) such as Mercurial and Git, it doesn't have a way to filter or organize the list of items, and it can only work with one project at a time.
Project Explorer is a VFPX project that replaces the Project Manager with a modern interface and modern capabilities. It has most of the features of the Project Manager but adds integration with DVCS (including built-in support for FoxBin2PRG and optional auto-commit after changes), support for multiple projects within a "solution," allows you to organize your items by keyword or other criteria, and has support for easy "auto-registering" addins that can customize the appearance and behavior of the tool.
See ProjectExplorer.pdf for documentation on Project Explorer.
A video is available showing Project Explorer and how it works.