Tool to effectively maintain todo lists aimed at developers

https://swapoff.org/devtodo.html

Wonderful Sega Genesis/MegaDrive emulator

https://dgen.sourceforge.net/

KAME DHCP6 client, server, and relay

http://wide-dhcpv6.sourceforge.net/

Decode and diagnose sniffed DHCP packets

http://www.mavetju.org/unix/general.php

Disk Information Utility

https://diskinfo-di.sourceforge.io/

Diagram creation program, similar to Visio

https://live.gnome.org/Dia

Console Interface to configure ports

https://bitbucket.org/m1cro/d4p/

Console Interface to configure ports

https://bitbucket.org/m1cro/d4p/

Simple english/german dictionary

http://www.de.freebsd.org/~wosch/dict/dict.cgi

Dict protocol (RFC 2229) server

https://www.dict.org/

GNU diff utilities

https://www.gnu.org/software/diffutils/diffutils.html

Monospace bitmap font, primarily aimed at programmers

http://www.donationcoder.com/Software/Jibz/Dina/

Open source video codec from the BBC

https://dirac.sourceforge.net/

Graphical diff tool

http://freecode.com/projects/dirdiff/

Client for managing and downloading certificate revocation lists

http://www.gnupg.org/aegypten2

Frontend to xgamma

http://gnu.ethz.ch/linuks.mine.nu/displaycalibrator/

Distribute compilation of C(++) code across machines (gtk monitor)

https://distcc.github.io/

Extremely fast library for floating-point convolution

http://cr.yp.to/djbfft.html

DjVu base libraries and utilities

https://djvu.sourceforge.net/

FTP benchmark program

http://www.kegel.com/dkftpbench/

Another hyper make utility

https://www.openoffice.org/tools/dmake/

Portable debug memory allocation library

https://dmalloc.com/

X11 menu application designed for the dwm window manager

https://tools.suckless.org/dmenu

Tool for dumping DMI (SMBIOS) contents in human-readable format

https://www.nongnu.org/dmidecode/

Distributed.net distributed computing project client

http://www.distributed.net/

Lightweight DNS forwarder, DHCP, and TFTP server

https://www.thekelleys.org.uk/dnsmasq/doc.html

Simple sudo alternative to run commands as another user

https://github.com/slicer69/doas/

Meta-port for the different versions of the DocBook DTD

http://www.oasis-open.org/docbook/

DocBook SGML DTD

http://www.oasis-open.org/docbook/

DocBook SGML DTD into nroff/troff -man macros converter

https://www.oasis-open.org/docbook/tools/dtm/

Generates various output formats from DocBook SGML documents

https://www.sourceware.org/docbook-tools/

DocBook XML DTD

https://www.oasis-open.org/docbook/

DocBook W3C XML Schema

https://www.oasis-open.org/docbook/xmlschema/1.0b1/

XSL DocBook stylesheets

http://docbook.sourceforge.net/projects/xsl/

Namespace-aware DocBook XSLT stylesheets

https://docbook.sourceforge.net

Emacs lisp modules for editing DocBook documents

http://nwalsh.com/emacs/docbookide/

DOCSIS(tm) binary configuration file encoder/decoder

https://github.com/rlaager/docsis

Command-line DNS client

https://dns.lookup.dog/

DNS-over-HTTP server proxy

https://github.com/DNSCrypt/doh-server

Doom data files (Doom, Doom II, Hexen, Heretic and Strife)

Complete Doom-based game IWAD that is Free Software

http://freedoom.sf.net/

Improved and extended version of Doom

https://doomlegacy.sourceforge.net/

Emulator of a PC with DOS

http://dosbox.sourceforge.net/

Simple, powerful configuration-file parser

https://github.com/williamh/dotconf

Binary-decimal and decimal-binary routines for IEEE doubles

https://github.com/google/double-conversion

Doulos SIL TrueType font collection

http://scripts.sil.org/cms/scripts/page.php?item_id=DoulosSILfont

Doulos SIL TrueType font collection, compact variant

http://scripts.sil.org/cms/scripts/page.php?item_id=DoulosSILfont

Secure, fast and powerful IMAP and POP3 server

https://www.dovecot.org/

Sieve plugin for the Dovecot 'deliver' LDA and LMTP

https://wiki2.dovecot.org/Pigeonhole

Documentation system for C, C++, and other languages

https://www.doxygen.org/