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.
draw.io: 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.