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 IDE for AROS Research Operating System
Test&Go Developer is a graphical IDE dedicated to the validation and processing of databases and files (XML, XLS, etc.). It creates platform independent scripts which you can configure through python events. It provides a complete plugin interface.
Open-jACOB is a rapid application development tool base on Eclipse that enables the development of RichInternetApplications that are based entirely on Java. Open-jACOB application can be deployed as a stand alone app's in a Web container like Tomcat.
UML# modelling tool written in and for C#. Support of patterns and templates for class diagrams.