AddOn Studio for World of Warcraft is an open source development tool for building AddOns for World of Warcraft.

IronPython development environment running as VS Shell Isolated application

A community project containing the source code and tests of a library for creating project system plug-ins for Visual Studio 2008 and upwards using C#.

Snippet Designer is a Visual Studio plug in which allows you to create, edit and search for snippets inside the IDE.

Real Script IDE makes it easier for UDK users to develop and debug UDK scripts. It's developed in C# with using Visual Studio Shell.