Available for download, both WSS 3.0 SDK and MOSS 2007 SDK Updates. I really love MS Download notifications

Windows SharePoint Services 3.0: Software Development Kit (SDK)
The Windows SharePoint Services 3.0 software development kit (SDK) contains conceptual overviews, programming tasks, samples, and references to guide you in developing solutions based on Microsoft Windows SharePoint Services 3.0. Read more and download here. Or You can also view the Windows SharePoint Services 3.0 SDK online in the MSDN Library.

SharePoint Server 2007 SDK: Software Development Kit
The Microsoft Office SharePoint Server 2007 SDK contains conceptual overviews, “How Do I…?” programming tasks, developer tools, code samples, references, and an Enterprise Content Management (ECM) starter kit to guide you in developing solutions based on Microsoft Office SharePoint Server 2007. Read more and download here. Or you can also view the Microsoft Office SharePoint Server 2007 SDK in the MSDN Library.