Finch
Based on libpurple, Finch is developed by the Pidgin project, and it pretty much supports the same features of it, except for the graphical part, of course. There are many chat protocols which it supports, including AIM, IRC, MySpaceIM, WLM, SILC, Yahoo! or ICQ.

Finch allows you to change the status, report idle time based on keyboard activity or turn it off, it supports plugins, file transfers, sounds, status messages, chat timestamps, customization of the contact list. In addition, it also lets you change and remembers the position and size of the windows.

The very nice thing is that once you get to know how to use it, Finch becomes a great tool for getting the job done.

Finch plugins

Several plugins can be configured and some of the plugins that come bundled with Finch by default include:

• Autoaccept – auto-accept file transfer requests from selected users
• Buddy Notes – store buddy notes
• Grouping – provides alternate ways to group buddies in the contact list
• Join/Part Hiding – hide join/part messages in large rooms

For using Finch you can find a comprehensive guide that I wrote a while ago following this tutorial. This page on the official homepage may be of help too.

Some of the basic keyboard shortcuts include Alt+Q to quit, Alt+A to open the options window, Alt+N to switch to the next window, Alt+C to close current window, Alt+R to resize a window, Alt+M to move a window, Tab to switch through options and Space to tick/untick an option.

CenterIM
CenterIM is another powerful IM client which is being actively developed, and supports the following protocols: ICQ, Yahoo!, WLM, AIM, IRC, Jabber, Gadu-Gadu and LJ.

When it starts, CenterIM shows a configuration window with various general options that can be changed or toggled by pressing Enter. The window that follows allows you to set up accounts for all the supported protocols.

CenterIM provides a default interface with the contact list to the left, and the discussion windows to the right. The online contacts are separated from the offline ones. I tried it using the Yahoo! protocol, and the contact groups seem to be ignored. After writing the text you want to send, press Ctrl+X to send it. Use the Escape key to switch to the contact list and Q to quit CenterIM.

Configuration window

CenterIM supports aways messages, anti-spam features, windows size configuration, keyboard binding, logs, aways system.

One feature which I found to be great is the possibility to enable Emacs/Vi keyboard bindings in the text editor, this making it easy for a person who is used to one of these ways of text input.

finch
Finch comes bundled with Pidgin, the popular IM client. Finch offers the same functionality that Pidgin offers, only that it does it in a terminal by using the ncurses library. It supports IM protocols like Yahoo, Google Talk, XMPP (Facebook), WLM (Windows Live Messenger) and more. A while ago I put up a detailed guide to Finch, which you can read here.
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htop
htop is an interactive process viewer tool using ncurses which has the great benefit that it allows to scroll up and down the list of processes, and it also uses graphs and colors. I think all these make htop a real gem for the Linux user.
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irssi
Very powerful IRC (Internet Relay Chat) client with an ncurses-based interface, implements multi-server support, can be expanded with Perl scripts, supports themes, DCC chat and every other possible feature IRC servers allow.
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mc
Midnight Commander is the famous twin-panel file manager for the Linux terminal, also based on ncurses and with lots of features.
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lynx
lynx is a popular web browser for the terminal. It supports protocols like HTTP, FTP or Gopher.
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gzip
gzip is a command-line tool for compressing and uncompressing files. All the files that end in a .tar.gz extension are archived with tar and compressed with gzip.
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bzip2
Same as gzip, bzip2 is a data compressor which takes a longer time to compress and uncompress files, but it also provides a more efficient compression algorithm which results in smaller files.
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tar
This tool is used to create archived files, and will also work in conjunction with gzip or bzip2. The command to compress a file or folder to gzip is (what follows after # is a comment):

tar -czf output_file.tar.gz input_file # compresses the file input_file into output_file.tar.gz
tar -czf output_file.tar.gz input_dir # compresses the directory input_dir to output_file.tar.gz
tar -czf output_file.tar.gz file1 file2 file3 # compresses file1, file2 and file3 to output_file.tar.gz

The c argument means compress, the z specifies the type of file to create (gzip file in this case), and f specifies the output file name. The command to uncompress a .tar.gz file:

tar -xzf input_file.tar.gz

This will extract the contents of input_file.tar.gz to the current working directory. Here are the commands for bzip2:

tar -cjf output_file.tar.bz2 input_file # compress input_file to output_file.tar.bz2
tar -xjf input_file.tar.bz2 # will extract input_file.tar.bz2 to the current directory

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aaxine
aaxine is a video player for console based on xine-lib multimedia player, using ASCII characters for video output. In Ubuntu it is provided by the xine-console package.
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…it’s Big Buck Bunny by the way

aview
This tool allows to view images as ASCII art. It only supports the following formats, which are either in binary form or ASCII plain text: PNM, PGM, PBM, PPM. It also supports FLI and FLC video formats.
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mencoder
mencoder is a powerful video encoder and decoder included in MPlayer and can convert between various video files.
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ffmpeg
Using libavcodec, ffmpeg is yet another powerful tool to encode/decode, record and stream video files.
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convert
Included in ImageMagick, convert is a tool that can convert between image formats, and also apply various effects to images or edit certain aspects, including resizing, cropping, blur or dither effects. convert is also used on web servers for image processing.
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moc
moc (music on console) is yet another ncurses-based music player for the terminal which plays formats like Ogg Vorbis, FLAC, MP3, WAV or WMA. It also supports themes and searching for files.
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Pidgin
This is the well-known IM client for the GNOME desktop environment with support for a wide variety of IM protocols, including AIM, Google Talk, ICQ, MSN or Yahoo. Piding also comes with a basic IRC client, support for multiple accounts, logging, system tray integration, themes, plugins.

Empathy
This is yet another GNOME instant messaging client with support for Google Talk, MSN, IRC, Salut, AIM, Facebook, Yahoo, Gadu-Gadu, Groupwise, ICQ and QQ. You will have to install specific Telepathy Connection Manager components in order to support all protocols e.g. sudo apt-get install telepathy-haze. Empathy features popup and sound notifications, smileys, spell-checking and themes.

Kopete
Kopete is the default chat client in KDE, offering support for a all the popular protocols (including IRC), tray integration, notifications, multiple accounts, tabbed chat windows, smileys, logs, video support and plugins.

Finch
Included in the Pidign package, Finch is a text user interface IM client which runs in a terminal and is based on libpurple, the same library Pidgin uses. This means Finch supports all the protocols that Pidgin has support for, including Yahoo, WLM, IRC or Google Talk. A full guide to Finch is located on the TuxArena Blog, here.

Emesene
Emesene is yet another powerful WLM client for GNOME with support for plugins, themes, notifications, webcam, configurable interface.

KMess
KMess is a WLM client for KDE with support for notifications, message stacking, avatars, logging, chat styles, emoticons, mail integration.

Ayttm
Ayttm is a graphical messaging application with support for Jabber, MSN, Yahoo, IRC and AIM. It features tabs, smileys, sound notifications, logging system.

Skype
Although Skype is renowned for being one of the most popular voice calls application, it also supports instant messaging, file transfers and webcam. Skype is closed-source and the official download page offers packages for all the major distributions out there.

CenterIM
This is a fork of CenterICQ, a terminal-based client for popular IM protocols. CenterIM supports ICQ, Yahoo, MSN, AIM, ICQ, Jabber, IRC, Live Journal and Gadu-Gadu. A feature which I found very useful was the possibility to enable Emacs or Vi bindings in the text editor, making it easier for persons who are used to these two IDEs to write text in a faster way.