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.
Geany is a small and lightweight integrated development environment. Basic features are syntax highlighting, code completion, code folding, call tips, many supported filetypes.
A free, portable, fast and simple C/C++ IDE
A new and improved fork of Bloodshed Dev-C++
Bluefish is a powerful editor for experienced web designers and programmers. Bluefish supports many programming and markup languages, but it focuses on editing dynamic and interactive websites.
Dev-PHP is a well-featured IDE for PHP. Fast and powerful, it is easy to use and has a lot of cool features : class browser, scripts preview, integration with PHP parsers, PHP-GTK library, and Xdebug of course. Dev-PHP-usb slips even into your pocket.
A powerful editor and graphical debugger for code written in Scilab
Scipad is a powerful editor and graphical debugger for programs written in the Scilab language. It is a mature and highly configurable editor, almost entirely written in Tcl/Tk. Scipad is a free software running on Linux and Windows. Licence is GPL V2.