htop is an improved version of top, a complex process viewer which allows to visualize processes in real time, see memory and CPU consumption, send signals to processes, renice processes, and sort them by various options. It makes use of colors and you can also scroll horizontally and vertically to see the full names of the running processes.

This is an ASCII image viewer for the terminal with support for all the popular image formats like JPG, PNG, GIF or BMP.
Homepage

Lynx is a well-known browser for the terminal.
Homepage

Other alternatives: w3m, links, elinks

Based on Xine, aaxine is an ASCII video player with many capabilities. An alternative to this is also mplayer, when running something like mplayer -vo caca video.avi.
Homepage

Nano is more user-friendly rather as feature-rich and powerful as Vim or Emacs, but it has support for syntax highlighting and it can be configured via the ~/.nanorc configuration file. Nano comes installed by default on most systems.
Homepage

Other alternatives: vim, emacs –no-window

CMus is a powerful player using ncurses with Vi-style keyboard shortcuts, seven different view modes, support for music library. It has shortcuts for virtually anything, tab-completion support, support for themes. CMus is a real gem for the audiophile that wants to use a terminal-based application for listening to music.
Homepage

Other alternatives: mp3blaster, moc, herrie, ogg123, mpg123, sox (play), nmpcpp

ctorrent is a command-line client for the BitTorrent protocol.

Other alternatives: rtorrent

Finch is included in the Pidgin package, using libpurple and ncurses, with support for a big number of protocols, including Yahoo!, AIM, IRC, or WLM. You can press Alt+A to open the preferences menu, Alt+R and Alt+M to resize and move windows, Alt+Q to quit, TAB and Space to navigate within windows.
Homepage

Irssi is probably the most popular IRC client nowadays, with lots of features, great flexibility, support for multi-networks, Perl scripting and whatnot. It’s complete and a very handy tool at all times.
Homepage

Midnight Commander, or mc for short, is a powerful twin-panel file manager that runs in your terminal, using the ncurses library.
Homepage

Other alternatives: vifm, lfm

This is the sound mixer from the ALSA package.
Homepage

iptraf is a powerful network monitor which allows you to select which interface to observe, show statistics, ingoing and outgoing traffic, open connections.
Homepage

Other alternatives: netstat

Mutt is a powerful email client with support for the POP and IMAP protocols, colors, PGP, configurable key bindings.
Homepage

Part of MPlayer, mencoder converts between various video files like AVI, Ogg, VOB or MPG.
Homepage

Other alternatives: ffmpeg

Emacs is a very powerful tool which can also run in text-mode by invoking it with the –no-window argument. Emacs is a development environment, as well as an email reader or file manager, and can be extended with scripts. As an text editor, Emacs comes with support for many languages, including C or Bash, syntax highlighting, check-spelling, indentation and much, much more. Emacs makes use of powerful commands and keyboard shortcuts designed to be faster when navigating through a text file.
Homepage

Alternatives: vim

dd is a tool that creates files using a specified filesystem, and it can also be used to create bootable USB devices from distribution ISO images, if used like sudo dd if=/path/to/image.iso of=/dev/sdX bs=4096k.

oggenc allows to encode WAV files to Ogg Vorbis format, specifying bitrate or quality.

Other alternatives: flac, lame, abcde

genisoimage allows you to create ISO filesystems from files and directories.
Homepage

convert allows to crop, resize, and convert various image formats. convert is part of the ImageMagick package.
Homepage

tar is a file archiver which can work in conjunction with gzip and bzip2 to create compressed files.
Homepage