software for Unix, &c.

Only free software is listed on this page, unless otherwise marked (≠).

command-line interface

These are command-line tools (though some may also have graphical interfaces, and specific support for other operating systems).

unfiled

Remind
language and interpreter for creating reminders and calendars
TestDisc & PhotoRec
data recovery software
HTTrack
website mirroring; a welcome alternative to wget
Plan 9 from User Space
the successor to Unix, ported to Unix
LillyPond
high-quality music printing (for European classical notation)
xdotool
simulate keyboard/mouse input, move/resize windows, &c.
PowerTOP
measure power consumption of running software

text

mawk
a very fast (if not the fastest) implementation of awk
Pandoc
convert from one markup format to another
peg/leg
recursive-descent PEG parser generators for C
ANTLR
ANother Tool for Language Recognition (also: ANTLRWorks GUI)

audio/video

SoX
digital audio conversion tools (with accurate resampling)
FLAC
Free Lossless Audio Codec
abcde
comprehensive CD encoder
mpg123
small, fast mp3 console-based player for files and streams
Music Player Daemon
audio server
StreamRipper
save mp3 (and ogg) streams to file
FFmpeg
record, convert and stream audio and video
MPlayer & MEncoder
movie player & encoder
VLC
multimedia player/server

pixels

gphoto
manipulate digital cameras
GraphicsMagick
image manipulation tools (originally forked from ImageMagick)

vector graphics

Asymptote
vector graphics language that provides a natural coordinate-based framework for 2D and 3D technical drawing
Context Free
environment for editing, rendering, and exploring images generated from design grammars

X window system

The following software is intended to be used under the X window system (though command-line interfaces are usually available, and some software may also run on other systems).

Code Browser
a fast/functional/clean folding text editor (supports elastic tabstops)
Claws Mail
fast/lightweight and extensible e-mail client
xxxterm
minimal Webkit browser with built-in cookie/script control
Transmission
BitTorrent client
MuPDF
small/fast PDF (& XPS) viewer
Evince
document (PDF, &c.) viewer (when MuPDF is not enough)
Pd (Pure Data)
real-time graphical programming environment for audio, video, and graphical processing
SuperCollider
environment and programming language for real time audio synthesis and algorithmic composition
Ardour
digital audio workstation
gmusicbrowser
music browser/player for large collections
EasyTAG
tag viewer/editor for most audio file formats
darktable
photography workflow application; enhance/develop raw images
AutoKey
desktop automation utility (text substitution, hotkeys, scripts)
Fontmatrix
font manager
FontForge
outline font editor
Microsoft ClearType fonts
≠ : a script to install the excellent fonts included with Windows Vista