"ma" is a minimalistic clone of the acme[1] editor used in Plan 9, and is written in Tcl/Tk. It has been tested with Tcl/Tk 8.5, mostly
under Linux. "ma" has successfully been run on Mac OS X with XQuartz, but needs a tiling window manager to be used in a satisfactory way.

Description of an interesting operating system (not implemented yet) based on ideas from QNX and Plan 9.

Apart from the Scheme compiler Chicken
 http://www.call-cc.org/
two other Scheme and one Forth system, there is
 MA, a minimalistic variant of the Acme editor, written in Tcl/Tk.

Beginning with the Eighth Edition, ed(1) had the following command.
This is from the Plan 9 ed man page.

         (.,.)b[+-][pagesize][pln]
              Browse.  Print a `page', normally 20 lines.  The
              optional `+' (default) or `-' specifies whether the
              next or previous page is to be printed.  The optional
              pagesize is the number of lines in a page.  The
              optional `p', `n', or `l' causes printing in the speci-
              fied format, initially `p'.  Pagesize and format are
              remembered between `b' commands.  Dot is left at the
              last line displayed.

Plan 9 from User Space moved from http://swtch.com/plan9port here. See also the Google discussion group https://groups.google.com/forum/#!topic/plan9port-dev/OqSvUDfwS-M.

9front is a fork of the Plan 9 operating system.

Foundation is a preservation system for users’ personal, digital artifacts. Foundation preserves all of a user’s data and its dependencies—fonts, programs, plugins, kernel, and configuration state—by archiving nightly snapshots of the user’s entire hard disk. Users can browse through these images to view old data or recover accidentally deleted files. To access data that a user’s current environment can no longer interpret, Foundation boots the disk image in which that data resides under an emulator, allowing the user to view and modify the data with the same programs with which the user originally accessed it.

Ugarit is a backup/archival system based around content-addressible storage. This allows it to upload incremental backups to a remote server or a local filesystem such as an NFS share or a removable hard disk, yet have the system instantly able to produce a full snapshot on demand rather than needing to download a full snapshot plus all the incrementals since. The content-addressible storage technique means that the incrementals can be applied to a snapshot on various kinds of storage without needing intelligence in the storage itself - so the snapshots can live within Amazon S3 or on a removable hard disk. Similar to Plan 9's venti, but implemented in Chicken Scheme. Encrypting the backup store (called vault) is supported.

Links to the single UNIX Specification (various versions).

9atom augments the plan 9 distribution with the addition of nupas (new mail system), extra pc hardware support, IL and Ken's fs.

The CSS stylesheet might be interesting.