Data compression library with low memory usage

https://www.oberhumer.com/opensource/ucl/

Command-line tools for building TCP client-server applications

https://cr.yp.to/ucspi-tcp.html

DNS resolver library with sync and async queries

https://www.corpit.ru/mjt/udns.html

Fast copy utility for UFS filesystem

http://people.freebsd.org/~simokawa/ufs/

HTTP web service mocking library for GNOME

https://gitlab.com/groups/uhttpmock

Convert MagicISO UIF files to ISO9660

http://aluigi.altervista.org/mytoolz.htm

Ultra lightweight xml-rpc library written in C++

http://ulxmlrpcpp.sourceforge.net/

Extract, view & test ACE archives

http://www.winace.com/

Allows files to be extracted from ARJ archives

http://www.arjsoftware.com/

Validating, recursive, and caching DNS resolver

https://www.nlnetlabs.nl/projects/unbound

Remove idle cursor image from screen

Unicode emoji data files

http://www.unicode.org/emoji

Library for single instance applications

http://live.gnome.org/LibUnique

Convert ASCII newlines between CR/LF and LF

ODBC library suite for Unix

http://www.unixodbc.org/

Extract Makeself archives

Convert any document from and to any LibreOffice supported format

http://dag.wieers.com/home-made/unoconv/

Extract, view & test RAR archives

https://www.rarlab.com/

Lists/Extracts files from a "*.tgz" or "*.gz" archive

List, test, and extract compressed files from a ZIP archive

http://infozip.sourceforge.net/UnZip.html

Zoo archive extractor

http://freshmeat.sourceforge.net/projects/unzoo/

D-Bus daemon for simplifying power management tasks

http://upower.freedesktop.org/

APC Smart UPS Monitoring Daemon

Ultimate Packer for eXecutables

https://upx.github.io/ https://github.com/upx/upx

URI parsing library

https://github.com/uriparser/uriparser

URL extractor/launcher

https://github.com/sigpipe/urlview

Database of all known IDs used in USB devices

http://www.linux-usb.org/

Daemon for multiplexing connections over USB to iPhone/iPod Touch

https://www.libimobiledevice.org

FreeBSD userboot.so for bhyve

UTF-8 processing library

https://juliastrings.github.io/utf8proc/

hash table for C structures

http://uthash.sourceforge.net/

Library for uu/xx/Base64/BinHex/yEnc de-/encoding

http://www.fpx.de/fp/Software/UUDeview/

Video4Linux IOCTL header files

https://linuxtv.org

Open source JavaScript engine by Google

https://v8.dev/

Programming language and compiler that converts Vala code into C code

https://wiki.gnome.org/Projects/Vala

Memory debugging and profiling tool

https://www.valgrind.org/

Markup Validation Service from the World Wide Web Consortium (W3C)

https://validator.w3.org/

High-performance HTTP accelerator

http://www.varnish-cache.org/

Open source Gameboy Advance emulator

http://vba.ngemu.com/

GNU VCDImager/VCDRip -- The GNU VideoCD Image Maker/Ripping Tool

https://www.gnu.org/software/vcdimager/

Graphical class browser for Xt

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

Web-based Version Control Repository Browsing

http://www.viewvc.org/

Improved version of the vi editor (x11 flavor)

https://www.vim.org/

VNC server that allows for remote access to your GNOME desktop

https://wiki.gnome.org/Projects/Vino

vitetris is a terminal-based Tetris clone by Victor Nilsson.

http://victornils.net/tetris/

Locks a terminal

http://freshmeat.net/projects/vlock

Management system for bhyve virtual machines

https://github.com/churchers/vm-bhyve

Captures a VNC desktop session and saves as a flv

http://www.unixuser.org/~euske/python/vnc2flv

Fast offline password cracker for VNC passwords

http://www.randombit.net/projects/vnccrack/

Multi-client VNC proxy server

https://sourceforge.net/projects/vnc-reflector/