SharePoint Server 2007 SDK
The SharePoint Server 2007 SDK is a software download which add templates like the SharePoint Workflow Sequential Library when creating a workflow solution in Visual Studio.
As Microsoft reports on their download page:
Brief Description
The Microsoft Office SharePoint Server 2007 Software Development Kit (SDK) contains conceptual overviews, programming tasks, code samples, references, and an Enterprise Content Management (ECM) starter kit to guide you in developing solutions based on Microsoft Office SharePoint Server 2007.
Other notes on the download page from Microsoft:
Overview
The Microsoft Office SharePoint Server 2007 SDK is designed for solution providers, independent software vendors, value-added resellers, and other developers to learn about the new Office SharePoint Server 2007 enterprise application and platform. It features conceptual and "How to" articles, sample code, and programming references.
The download file OfficeServerSDK.exe provided for the SharePoint Server 2007 SDK contains the following:
Compiled HTML Help files (updated with new content since RTM), OSSSDK2007.chm and WSS3sdk.chm, which contain conceptual topics, Class Library and Web Service References for SharePoint Server 2007 and Microsoft Windows SharePoint Services 3.0.
Code samples that relate to topics in the SDK: Business Data Catalog metadata samples, Custom Single Sign-On Provider, SampleWebService, and Custom Filter Web Parts. The sample projects use Microsoft Visual C# and Microsoft Visual Studio 2005.
Enterprise Content Management Starter Kit.Also included in the SharePoint Server 2007 SDK, the Enterprise Content Management (ECM) Starter Kit is designed for solution providers, independent software vendors, value-added resellers, and other developers to learn about how to extend the new ECM features and platform in Office SharePoint Server 2007. It contains the following:
Download the file here.
