cmus
cmus is a music player that I admire the most when it comes to command-line because it’s really powerful and has a lot of nice features. It is built with ncurses and therefore providing a text-user interface. cmus is indeed feature-rich, with several view modes and Last.fm song submission support via scripts. It supports Vi-like commands and auto-completion with Tab too. Recently I wrote a full guide on how to use cmus, you can read it here.
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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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