You can download the SDK here. This is the second release of the Visual Studio 2005 SDK (version one was in November 2005). From the download description: "The April v2 RTM is the next milestone in the VS 2005 SDK. This RTM release contains updated VS Integration sample and documentation content, including updates to the IronPython end-end integration sample with Windows Forms Designer integration, updated C# and C++ Editor Reference samples, new examples (CustomSearchProvider, RegExLanguageService, XMLDesigner, LanguageInADialog, CodeSweep, ToolboxControlsInstaller, SingleFileGenerator), addition of Shell, LanguageService, and Project MPF source code, DSL Tools extensibility, updates to VSTS content to match RTM, and signed VSTA bits."

Enjoy!