Simple window manager with no fat library dependencies

https://www.nongnu.org/ratpoison/

Distributed, collaborative, spam detection and filtering network

http://razor.sourceforge.net/

Unix incarnation of the plan9 shell

http://tobold.org/article/rc

Checks rc scripts for style and common errors

https://github.com/crees/rclint

Sync files to and from various cloud services

https://rclone.org/

Version control system (as was in FreeBSD prior to removal)

http://www.gnu.org/software/rcs/

RDP client for Windows NT/2000/2003 Terminal Server

http://www.rdesktop.org/

Fast C++ regex library

https://github.com/google/re2

Lexer generator for C, C++ and Go with a focus on generating fast code

https://re2c.org/

Library for editing command lines as they are typed

https://tiswww.case.edu/php/chet/readline/rltop.html

Kernel driver for Realtek PCIe Ethernet Controllers

https://www.realtek.com/en/component/zoo/category/network-interface-controllers-10-100-1000m-gigabit-ethernet-pci-express-software

Build-tool for Erlang that follows OTP principles

https://github.com/rebar/rebar

Build tool for Erlang

https://www.rebar3.org/

Converts files between character sets and usages

http://www.gnu.org/software/recode/recode.html

Persistent key-value database with built-in net interface

https://redis.io/

Persistent key-value database with built-in net interface

https://redis.io/

High-level interface for RDF

http://librdf.org

Microsoft .Net Portable Class Library Reference Assemblies

https://blogs.msdn.microsoft.com/dotnet/2013/10/14/portable-class-library-pcl-now-available-on-all-platforms/

Graphical interface to rdesktop

http://gap.nongnu.org/remotedesk/index.html

Fast, secure, efficient backup program

https://restic.github.io

Uncompile an rgb color-name database

https://gitlab.freedesktop.org/xorg/app/rgb

Utility and library for computing and checking of file hashes

http://rhash.sourceforge.net/

Audio player for GNOME

https://wiki.gnome.org/Apps/Rhythmbox

Image viewer with Xfce integration

https://goodies.xfce.org/projects/applications/ristretto

Flexible message logging facility for C++

https://www.arg0.net/rlog

Roboto typeface family

http://developer.android.com/design/style/typography.html

Fully skinnable XHTML/CSS webmail written in PHP

https://roundcube.net/

Convert .rpm files to cpio format

The RPM Package Manager

https://rpm.org/

Round Robin Database Tools

http://www.rrdtool.org/

Fast spam filtering system

https://rspamd.com/

Restricted Secure SHell only for sftp or/and scp

http://www.pizzashack.org/rssh/index.shtml

Network file distribution/synchronization utility

http://rsync.samba.org/

Kernel module which provides /dev/rtc device support

RTMP streams download utility

http://rtmpdump.mplayerhq.hu/

BitTorrent Client written in C++

https://github.com/rakshasa/rtorrent

Ruby interface to Oracle Berkeley DB revision 2 or later

https://github.com/knu/ruby-bdb

Binary data dumper module for Ruby

http://www.geocities.co.jp/SiliconValley-Oakland/2986/

Package management framework for the Ruby language

https://rubygems.org/

Object-oriented interpreted scripting language

https://www.ruby-lang.org/en/

Object-oriented interpreted scripting language

https://www.ruby-lang.org/en/

Object-oriented interpreted scripting language

https://www.ruby-lang.org/en/

Fast, open source text processor and publishing toolchain

https://asciidoctor.org/

Tool that manages gem dependencies for Ruby applications

https://bundler.io/

Modern concurrency tools for Ruby

https://github.com/ruby-concurrency/concurrent-ruby

Fast, Enjoyable HTML Parser for Ruby

https://github.com/hpricot/hpricot

New wave Internationalization support for Ruby

https://github.com/ruby-i18n/i18n

Library for handling money and different currencies

http://rubymoney.github.io/money/

Framework-agnostic way to render logic-free views

https://github.com/mustache/mustache

Fast Markdown converter for ruby based on discount

https://dafoster.net/projects/rdiscount/