DXCore for Visual Studio 2005
DXCore is a powerful and easy to use extensibility framework for Visual Studio® .NET. In the interest of encouraging developers to be pro-active about productivity, Developer Express is making available for general download DXCore for Visual Studio® .NET so that developers can build productivity plug-ins FOR PERSONAL USE ONLY
http://www.devexpress.com/Downloads/NET/IDETools/DXCore/
I haven't looked in depth at how good DXCore is but browsing www.devexpress.com it looks like a few powerful addins have be created using it, so it must be alright.