Subversion's modular design (covered in the section called “Layered Library Design”) and the availability of language bindings (as described in the section called “Using Languages Other than C and C++”) make it a likely candidate for use as an extension or backend to other pieces of software. For a listing of many third-party tools that are using Subversion functionality under-the-hood, check out the Links page on the Subversion website (http://subversion.tigris.org/project_links.html).