MPEG-2 and MPEG-4 AAC audio decoder

https://github.com/knik0/faad2

Visual mail, user, and print face server

http://faces.sourceforge.net/

Functional Constraint Library

http://www.recherche.enac.fr/log/facile/

Simulate the root user behaviour

http://freshmeat.net/projects/fakeroot

File alteration monitor

http://oss.sgi.com/projects/fam/

Programming font, designed with functionality in mind

https://fontlibrary.org/en/font/fantasque-sans-mono

Serif based on the work of a famous Czech-American type designer

https://www.theleagueofmoveabletype.com/fanwood

Standard set of Unicode Persian fonts

http://www.farsiweb.ir/wiki/Persian_fonts

Collection of GStreamer modules and libraries for videoconferencing

https://www.freedesktop.org/wiki/Software/Farstream/

Fast and exact C++ number parsing library

https://github.com/fastfloat/fast_float

Like neofetch, but much faster because written in C

https://github.com/fastfetch-cli/fastfetch

version of JDK's `jar' command written entirely in C

http://fastjar.sourceforge.net/

Accuracy-focused XAudio reimplementation

https://fna-xna.github.io/

Functional programming language for realtime audio signal processing

https://faust.grame.fr/

FastCGI Development Kit

http://www.fastcgi.com/

Flexible Input Method Framework

https://fcitx-im.org/wiki/Fcitx

Chewing support for Fcitx

https://fcitx-im.org/wiki/Fcitx

Cloud-based Pinyin module for Fcitx

https://fcitx-im.org/wiki/Fcitx

Fcitx IM framework configuration tool

https://fcitx-im.org/wiki/Fcitx

Libpinyin support for Fcitx

https://fcitx-im.org/wiki/Fcitx

Fcitx support for Qt5

https://github.com/fcitx/fcitx-qt5

New implementation of the Fcitx IME framework

https://github.com/fcitx/fcitx5

Pinyin and table input method support for Fcitx5

https://github.com/fcitx/fcitx5-chinese-addons

Lua support for Fcitx5

https://github.com/fcitx/fcitx5-lua

Fcitx support for Qt (qt5)

https://github.com/fcitx/fcitx5-qt

Fcitx support for Qt (common files)

https://github.com/fcitx/fcitx5-qt

Other table support for Fcitx5

https://github.com/fcitx/fcitx5-table-other

CUI-based file and directory management tool

https://hp.vector.co.jp/authors/VA012337/soft/fd/

Port of the Fraunhofer FDK AAC Codec Library

https://sourceforge.net/projects/opencore-amr/

FDM Material database for 3D printers

https://github.com/Ultimaker/fdm_materials

Haptic/visual/audio feedback for GNOME

https://source.puri.sm/Librem5/feedbackd

Image viewer that utilizes Imlib2

https://feh.finalrewind.org/

Batch mail retrieval utility for IMAP/POP3/ETRN/ODMR

https://www.fetchmail.info/

Python-based GUI to configure fetchmail

https://www.fetchmail.info/

Realtime audio/video encoder/converter and streaming server

http://ffmpeg.org/

Realtime audio/video encoder/converter and streaming server (legacy 4.* series)

https://ffmpeg.org/

Lightweight video thumbnailer that can be used by file managers

https://github.com/dirkvdb/ffmpegthumbnailer

FFmpeg-based source library and VapourSynth plugin

https://github.com/FFMS/ffms2

FFmpeg version of Nvidia Codec SDK headers

https://github.com/FFmpeg/nv-codec-headers

Fast C routines to compute the Discrete Fourier Transform

http://www.fftw.org/

Fast Discrete Fourier Transform (Single Precision C Routines)

http://www.fftw.org/

Fast Discrete Fourier Transform (Long Double Precision C Routines)

http://www.fftw.org/

Integrated benchmark program

http://tromp.github.io/c4/fhour.html

Easy to use graphical editor for electronics

https://darwinne.github.io/FidoCadJ/index.html

Small, efficient C++ GUI library designed for games

http://fifengine.net/

Narrow monospaced fonts

https://fontlibrary.org/en/font/fifteen

Tools to convert Xfig .fig files

https://en.wikipedia.org/wiki/Xfig

Archive manager for zip files, tar, etc

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

Tcl/Tk-based filemanager with FTP capabilities

https://sourceforge.net/projects/filerunner/

Fast and reliable cross-platform FTP, FTPS, and SFTP client

https://filezilla-project.org/