Learn about workflow in Windows SharePoint Services version 3
Windows SharePoint Services version 3 is a useful option for a host for building workflows on Windows Workflow Foundation. They implement a host and their own persistence service and tracking service which are aligned with the SharePoint data store.
There's a whitepaper available describing both Windows SharePoint Services and Microsoft Office SharePoint Server.
My friend Ian Morrish has set up a Windows SharePoint Services demo site which has all sorts of useful things on it.
You can read the Windows SharePoint Services SDK here http://msdn2.microsoft.com/en-us/library/ms416312.aspx
See also the SharePoint home page, the SharePoint news groups, or the SharePoint support home.
Here's a great talk on building Windows SharePoint Services workflows in Visual Studio by Eilene Hao. I helped Eilene create this talk and it's a very powerful way to use WF in the hosting environment that SharePoint provides.
There's some other stuff here on MSDN to check out also for SharePoint which has lots of workflow in it.
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Edit: Here's more from Eilene, a series of articles on creating workflows in Visual Studio for Windows SharePoint Services.
