Hgfs is a read-only filesystem interface to Mercurial repositories. The interface gives access to the commit message, manifest, and files of each revision, and to .tgz's of each revision (the .tgz's are generated as they are read). The filesystem is a front-end for the Mercurial library that comes with it. All code is written in Limbo, for Inferno.
The J9P development framework is designed to ease the integration of legacy or non-9P services into 9P networks. Plan9 (and Inferno) introduced a paradigm for services in networked environments where every service is mapped to file operations in a virtual namespace that is published by the server that provides the service (either locally or remote). The J9P framework provides a generic 9P server that publishes such "virtual" namespaces to clients. J9P namespaces are defined in a configuration file and are assembled from directories (branches) and files (leaves).