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.
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.
Visual Lua is the Integrated Development Environment (IDE) for Lua language. You may use it for debugging the scripts written using wxLua library where the event-handling takes place. It supports multithreaded debugging as well.
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).