Tools for any programming language. Tools dedicated to a programming language are given in the cards of languages below...
We do use most of these tools in production: they are all free, easy to use, and improve productivity a lot.

Programming and data languages

Asm.js - Basic - C - C++ - C# - Dart - Go - Java - JavaScript - Julia - Pascal - PHP - Prolog - Python - Ruby - Rust - Scala - Scriptol - Swift - TypeScript - HTML - Wasm - XML - XAML - SQL

IDE & RAD (Integrated & Rapid Application Development)
NetBeans	IDE for several languages including Java and PHP with a built-in server. (Java)
Visual Studio Community	Free version of Visual Studio to build apps for all devices. (Win).
KDevelop	Made for Linux but ported to Windows and Mac thanks to Qt, it supports multiple languages: C++, Objective C, PHP, Python ... Version 5 features a CLang plugin. Warnings and syntax errors appear in the editor, with a suggestion to correct them.
Ideone	Interactive online tool that allows testing (compile/interpret) source code in 40 programming languages and display the result.
Sikuli	Created by MIT, a GUI automation and test tool. It simulates the interactions and allows to associate code to widgets.
	List of multi-languages IDEs.

Graphical User Interfaces for cross-platform programs
HTML 5 frameworks	Universal interface language. It can be used for local applications with Node.js, as demonstrated in the JavaScript section .
HTML 5 games engines	For the desktop, there is SDL (in the list below), but for the browser, more frameworks are offered (and free).
Qt	Cross-platform user interface framework, inclusing mobiles. LGPL license. (Win/Lin/Mac/Mobiles). An IDE, QtCreator is provided to build applications.
	List of all graphical user interfaces libraries.

Compilers and development tools
LLVM	Includes the Clang, C, C++ and Objective-C compiler, producing a portable bitcode, for its fast virtual machine. An optimizer at each stage. Can produce binary too. It may be complemented by other tools, such as Emscriptem that converts its bitcode to JavaScript.
Gow	Windows version of all Unix tools. (Win)
rr	A dreamed debugging tool from Mozilla: it saves the execution of a program and allows to replay the film, to find the moment when an error occurred. Currently work on Linux 32-bit only.
SWIG	Tool that generates interfaces for different languages so they may be used in conjunction with C++.

Utilities to releases and source documenting
Inno Setup	Complete installer, can add your program on the desktop, provides an uninstaller for your program, into the archive that is bigger than those of above. Open source.
IZarc	Archive manager. Can build, extract, convert ZIP, BZ2, TAR, GZ, CAB archives and the new 7z format. Manages also CD images
Doxygen	Produces a HTML/Latex/XML/Man documentation from source files.
sourceMakeup	PHP version of the Docco family, a tool ported to many programming languages, to display a source code in HTML with an improved presentation.
Omaha	Automatical update of software. Google open sourced its update system, used by Chrome.
GitHub Windows	To manage a open source project hosted on GitHub, a simplified manager for Windows. Similar tools exist for Linux. (Win/Lin).

Graphics
Paint.NET	Drawing program similar to Photoshop, easier to use than Gimp. Supports layers and a lot of filters. Has graph tools. (Win).
Gimp	Drawing program, open source replacement for Photoshop. Despite that it is bugged under Windows has interesting features. The filters provide tools for sophisticated image processing. (Win/Lin)
XnView	Image viewer and manager, recognize almost any format plus video ones: Mpeg, Divx (Win/Lin)
Inkscape	Vectorial graphical editor allowing to make SVG files (Win/Lin/Mac).
OpenGameArt	This site sponsored by Mozilla collects graphic elements such as sprites and scenery to help make games for free.
Terragen	Landscape editor with photographic quality.
	More graphic and drawing tools.

Graphics and Video: Expert level
Visualization Toolkit	3D visualization tool, open source. (Win/Lin).
POV	Most powerful 3D ray tracer. Programming skill required. (Win/Lin)
Open FX	3D image maker.

Web and server
Anaa	Small Ajax framework with the main functions: load XML, text or HTML contents, post a text or command to the server.
Link Checker	Check a website for broken links. (PHP). See also Xenu's link (Win)
List of CMS	Modern big websites are built with a CMS (Content Management System) as Joomla, Drupal, Xoops for portails, phpBB or punBB for forums, Wordpress for blogs.
FreeFTP	Upload files on the net. (Win). For updating a website from local files, the command line FTP synchronizer is well suited.
Page Test	Check your web pages for validity, according to the HTML 4 standard. You can check also XML and CSS.
Postman	Tests a Web service, and records your actions for a further use. It is an extension to install in Chrome.
	List of webmaster's tools that are needed to manage websites.

Editors (text, code, HTML, XML)
VS Code	From Microsoft, a simple code editor for all programming languages with highlighted syntax. Supports dual panels. Git. Special features for JavaScript and Node.js. (Win/Lin/Max).
PSPad	Complete editor for any programming languages including HTML and XML. Internet features. Several foreign languages supported. (Win).
Geany	Cross-platform tabbed code editor. Extensible with plugins.
Blue Griffon	WYSIWYG HTML editor. (Win/Lin/Mac).
Treesheet	Organizer which has the functions of a spreadsheet and those of an outliner. Only a programmer can use a tool like this. Open source. (Win/Lin/Mac).
Pandoc	Convert documents between different markup formats.
LibreOffice	Perfect to write manuals with images, supports HTML and PDF. (Win/Lin/Mac).
	List of web page editors.
More tools and infos on the HTML card. Languages may have specialized editors also.

Tools for SQL databases
Kexi	Database designer. (Win/Lin).
XAMPP	Local server with MySQL and user interfaces to databases. (Win/Lin/Mac)

Libraries and APIs
Libuv	Framework for asynchronous IO and data exchange. It is the base of Node.js, but can also be used with other languages ​​like C, Julia. Also manages the file system with a common interface for all OS. (Win/Lin).

Documentations and tutorials
OpenGL	All Open GL tutorials.
OpenGL Lessons	Complete tutorial on OpenGL with examples and sources for each compiler.
DaFont	Fonts and tools.
List of free books	Available on PDF file to download.
Cheat-sheets	List of reminders of any format or programming language.
See at each programming language on top of this page for specific tutorials.

Making money
Bountysource	Start a campaign to get funding with an open source software. Even a programming language may be founded (See Crystal, Nim).