"2007 Office System Starter Kit: Enterprise Content Management Starter Kit" includes a white paper on adding activities into SharePoint Designer 2007: http://www.microsoft.com/downloadsdefault.aspx?familyid=38ca6b32-44be-4489-8526-f09c57cd13a5&displaylang=en
Visual Studio 2005 Extensions for Windows Workflow Foundation download: http://www.microsoft.com/downloads/details.aspx?FamilyId=5D61409E-1FA3-48CF-8023-E8F38E709BA6&displaylang=en
"MVP-Submitted: The Power of Custom Workflow Activities (Part 1)" by Maurice de Beijer: http://msdn2.microsoft.com/en-us/vbasic/cc351048.aspx
White paper "Identifying the Workflow Potential of Business Processes" by Jörg Becker, Christoph v. Uthmann, Michael zur Mühlen, and Michael Rosemann: http://www.workflow-research.de/Publications/PDF/JOBE.CHUT.MIZU.MIRO-HICSS(1999).pdf
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