A free IDE for C#, VB.NET, & Boo projects on Microsoft's .NET platform
SharpDevelop is the open-source IDE for the .NET platform. Write applications in languages including C#, VB.NET, F#, IronPython and IronRuby, as well as target rich and reach: Windows Forms or WPF, as well as ASP.NET MVC and WCF. It starts from USB drives, supports read-only projects, comes with integrated unit and performance testing tools, Git, NuGet and a lot more features that make you productive as a developer.
Rapid applications development tool and libraries for FPC
The Lazarus IDE is a stable and feature rich visual programming environment for the FreePascal Compiler. It supports the creation of self-standing graphical and console applications and runs on Linux, FreeBSD, MacOSX and Windows.
An open-source x64/x32 debugger for windows.
An open-source x64/x32 debugger for windows. Development is done on GitHub (https://github.com/x64dbg/x64dbg). Daily snapshots are uploaded here, snapshots for every commit can be found on http://releases.x64dbg.com
DBG is a full-featured PHP Debugger and Profiler. Makes it easier to find problems or bugs in PHP scripts. DBG supports back-tracking, watching variables, viewing arguments passed to all nested function calls at any point of execution. Opensource.
Edyuk is a free, open source, fast and lightweight development environment which supports C++ and Qt 4 out of the box and can be extended through plugins. It works under all platforms supported by Qt 4 (Linux, Mac, Windows, BSD, ...).
psxdev.ru is home of the following projects: HLPSE (HLE PSX Emulator), HLPSD (HLE PSX DisASM), HLPStEst (HLE PSX Emulator (obsolete)), TIMGRAB (TIM extractor).
Denwer is a collection of often-used web-servers for web-programmers. It is used to debug web-sites under Win32. Denwer includes: friendly installer; Apache; PHP; MySQL; Perl; virtual host management system; phpMyAdmin; sendmail stub; start/stop manager.
A Finite State Machine Editor, written on QT. It allows to draw Finite State Machine with easy GUI and store it in XML file. There are Finite State Machine Compilers to translate this description to source code (technique like QT's UIC uses).