On behalf of the VS Tools Ecosystem team, I am pleased to announce that after months of hard work, VS SDK 4.0 has been released! This is the last SDK release planned for Visual Studio 2005. The team has done a ton of work to make this release friendly to developers who are new to Visual Studio extensibility. It is available for immediate download on the Microsoft Download Center.

Some of the new features included in this release:

• VS SDK Browser – the new entry-point to the entire SDK; includes new QuickStart Tutorials and a completely revamped sample browsing experience, community resources and tools info
• Package Load Analyzer – allows you to easily diagnose package load failures
• Toolbox Installer redistributable package – allows component vendors to simplify deployment, along with a sample demonstrating how to do it all. 
• Sandcastle – new set of tools for generating managed class library documentation

There's an article in eWeek for this release. (That's a first for the SDK as far as I know.)