Qt

Something something eating your own dog food. Listed in the official tools, but not listing it here doesn't feel right.

The closest thing to a standard C++ build system out there. The bundled GUI uses Qt.

But can it run Crysis? Some auxillary tools use Qt.

The most popular documentation generator for C++, though it supports other languages as well. The bundled GUI uses Qt.

Virtual tabletop for remote Role Playing Games.

Secure your conversations so well, you'll raise the blood pressure of potential eavesdroppers. The desktop client uses Qt and is open source.

Perhaps the world's most popular tilemap editor. Suitable for level design in games.

Realtime visual programming language for interactive media.

Sega Saturn emulator.

Slicer application used to prepare your 3D models for printing with a 3D printer. Built with Uranium framework which is based on Qt 5 and Python 3.

I've never heard of this language either, honestly.

Experimental wrapper for Qt that allows it to be used by .NET-based languages such as C#. Young, but active.

Cross-platform IRC framework. We've got enough IRC clients out there already, but I suppose this could also be used to implement a chat function in your application (e.g. for community support).

Web framework that takes some ideas from Perl's Catalyst framework.

Dice Roller framework based on its own syntax.

Binding for QML in C, primarily designed as an API for other languages to support QML.

Convert images from any Qt-supported format to OpenGL-format textures. Useful for speeding up texture loading in graphics applications.

Django-inspired text templating.

Dependency injection for Qt.

Metric ass-ton of libraries for KDE, but many of them can be used with regular Qt applications as well. Cross-platform, for the most part.

Embed a hex editor in your application.

Material Design implemented in QML.

Widget that lets you create and edit nodes in a graph similar to the kind provided by Unreal Engine, Substance Designer, or PureData.

Let your application update itself.

Fans of type theory will enjoy these sum and optional types.

Render Mustache templates.

Actively developed multimedia framework.

Powerful set of libraries that provide GUI, logging, testing, configuration, and project functionality (among other things) for your application.

Notification framework.

Make your application capable of anything.

Create regular expressions using natural English expressions. Part of the VerbalExpressions project, which does the same thing for a wide variety of languages.

If you need to write a web application server in Qt for some reason, this'll help you along.

Read and write spreadsheets in Microsoft Excel format.

Experimental library to write Qt applications without using the moc.

Another Qt-centric package manager. If you want to see the offerings without using QtPods, the full list is available here, and can be added to by submitting a pull request.

Overview of advanced GUI programming techniques for those familiar (but not satisfied) with the basics. The source code used in the book is available for free.

Not your daddy's Qt. Tips and tricks for the experienced. The source code used in the book is available for free.

Free book that focuses on QML, named for this town in northeastern Spain that the authors vacationed to once.

Free book created by Qt Company. It provides you with a walk through of the different aspects of application development using the new Qt 6. It focuses on the Qt Quick technology, but also provides necessary information about writing C++ back-ends and extension for Qt Quick.

There's no reason you can't use C++ libraries that weren't designed for Qt. Sometimes they might even suit your needs better than what Qt offers.

Same goes for C libraries; might wanna brush up on those RAII techniques while you're at it.

If you're not using qmake for your project, you're likely using CMake instead.

Qt is commonly used for graphics applications, even providing its own wrappers around OpenGL.