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.
StyxLib is a .NET implementation of the Styx (9P2000) protocol for distributed systems. This protocol is used by Inferno and Plan9 OSes. It is similar to other network file systems like NFS and CIFS, but in 9P systems, files do not always represent real files on disk, but can instead be interfaces to physical devices or programs.