Editors

FTE-Terminal
An editor reminiscent of the Borland programming environment.

Jed
Another easy-to-use editor.

joe
Easy to use editor, which can be used for development.

nano
Perhaps the easiest to use command ine text editor. They keybindings can be learned very quickly.

Vim
Widely used editor that supports all the common programming and scripting languages, and it has many advanced features for professional sysadmins.

XWPE
Usable but unstable editor, which is best used for the editing of standard text documents. Has a Curses-based interface.
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Office

abook
Address book. Contacts are saved, imported and exported as a text file. The interface is semi-graphical and intuitive. Apparently it can be integrated with the mutt client.
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Apline
Email client that's easy to use. Can be configured to use POP3 and IMAMP accounts. Some Web mail accounts can be accessed through local dedicated email client applications. Fairly straightforward to use.
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Calcurse
An organiser for the command line, which includes a calendar and To Do list. This integrates with Vim or nano for viewing and modifying notes.

sc
The Spreadsheet Calculator enables the creation, viewing and modification of spreadsheets, and has most the core features of a desktop spreadsheet application.

Utililties

Aptitude
A semi-graphical interface for the APT package manager. It enables the listing of installed and uninstalled components.

Midnight Commander
Semi-graphical file manager that allows the user to run other commands to perform file operations.

Other

Lynx
A command line Web browser that's easy to use and has all the core features of a desktop-based browser.
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MOCP
Media player, mainly for MP3 files.

Newsbeuter
RSS reader.

Olive
RSS reader.

Security

chkrootkit
Scans for rootkits by checking the integrity of core binary files on the system.
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Lynis
Another security auditing tool to replace the deprecated Tiger. Lynis scans the local system to determine its general level of security. A readable log file is generated during the scan, and it would make a good addition to a penetration testing report.
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rkhunter
The rootkit hunter. A larger package download that can complement chkrootkit and unhide.

unhide
Described as an open source forensics tool, unhide can be used to scan systems for rootkits. It can perform quick scans or comprehensive tests that compare process lists with procfs and memory allocations. The user can also selectively run specific tests.