abook: Address book application.

alpine: Email client.

Calcurse: Calendar and tasks application that's easier to use than taskwarrior, with a curses-based interface and simple keybindings. Can be configured to use any command line editor.

Lynx: Text-mode Web browser. This is particularly useful if you want to avoid downloading anything except the text content of a site, and have a more efficient Web browsing session.

Midnight Commander: One isn't limited to using the standard Linux commands when viewing directories and manipulating files. Midnight Commander is a semi-graphical directory and file browser with comprehensive features to automate tasks while allowing terminal commands to be run against the selected files or directories.

Nano: Perhaps the most accessible command line text and code editor available (it does syntax highlighting), though it lacks most the features of Vim.

Ranger: Another semi-graphical directory navigator.

sc: Spreadsheet application. It takes a little practice to start using.

tudu: A very straightforward and easy-to-use todo list application. Tasks can be presented in a tree view, given deadlines, categorised and marked.

Vim: By far my favourite editor, which is almost perfect for drafting blog posts, software development work and modifying configuration files. Vim is highly customisable, provides syntax highlighting, has a large collection of plugin scripts and even an option for encrypting content.

WordGrinder: Word processor application that enables documents to be exported to a number of common formats. Useful for drawing schematics, system diagrams, UML and any graphical information to be included in technical documents.

LINQpad: Editor for creating and modifying LINQ queries.

MySQL Workbench: A very comprehensive application for working with MySQL databases, remote and local.

Notepad++: A highly customisable and extendable text/code editor, with many useful plugins available.

SQL Server Management Studio: Microsoft's comprehensively-featured developer environment for working with MS-SQL databases.

Visual Studio: The main developer environment for the .NET framework and associated programming languages.

Visual Studio Code: Editor and developer environment with numerous extensions available for working with most languages, frameworks and database types. Mainly used for Web application development outside MVC / ASP.NET.