Visual Studio 2005 SDK version 4.0 is available for download.
The latest and potentially last version of the Visual Studio 2005 SDK's is out and available for download here.
Overview of the SDK (From the download site...)
This is the official Visual Studio 2005 software development kit that allows developers to integrate tools, editors, designers, languages, and much more inside Visual Studio 2005. It adds the following features: VS SDK Browser, Package Load Analyzer, Toolbox Control Installer together with a toolbox control installation sample, a redistributable package for Toolbox component vendors that simplifies deployment, a new tool for generating managed class library documentation (Sandcastle), an updated Setup experience, a new toolbar combo box sample, new integration tests for the EditorWithToolbox sample that use the VsIdeHostAdapter and much more.
~ Robert Shelton