Fuse4X allows you to extend OS X's native file handling capabilities via 3rd-party file systems. It can be used as a software building block other products.
As a user, installing the Fuse4X software package will let you use any 3rd-party file system written atop Fuse4X. As a developer, you can use the Fuse4X SDK to write numerous types of new file systems as regular user-mode programs. The content of these file systems can come from anywhere: from the local disk, from across the network, from memory, or any other combination of sources. Writing a file system using Fuse4X is orders of magnitude easier and quicker than the traditional approach of writing in-kernel file systems. Since Fuse4X file systems are regular applications (as opposed to kernel extensions), you have just as much flexibility and choice in programming tools, debuggers, and libraries as you have if you were developing standard OS X applications. In more technical terms, Fuse4X implements a mechanism that makes it possible to implement a fully functional file system in a user-space program on Mac OS X (10.5 and above). It provides API compatible with the FUSE (File-system in USEr space) API that originated on Linux. Therefore, many existing FUSE file systems become readily usable on OS X.
The Fuse4X software consists of a kernel extension and various user-space libraries and tools. It comes with C-based and Objective-C based SDKs. If you prefer another language (say, Python or Java), you should be able to create file systems in those languages after you install the relevant language bindings yourself.