Windows Workflow Foundation is a multi-faceted technology. Some people will look at WF and see a tool to manage long-running business processes, while others see a rules engine. Yet another perspective is to look at WF as a tool for building domain specific languages.
Last week I had numerous conversations with talented individuals who reinforced this theme of WF as a DSL tool. For example- Kathleen Dollard builds code-generation tools for developers, and Sam Gentile builds applications for financial analysts. Both of them are giving their end users the ability to create tailored software using a domain specific language. The DSLs themselves are composed from custom activities in Windows Workflow Foundation. Microsoft has built a few DSLs on top of WF too, most notably in the latest release of Sharepoint Server and in Speech Server 2007.
Here's a few interesting resources on custom activity development in WF:
Build Custom Activities To Extend The Reach Of Your Workflows – Matt Milner
Speaker Verification Activity for Speech Server 2007 – Casey Chestnut
Custom Activity Samples in the SDK
HOW TO: Create a custom Windows Workflow Activity and make it available in SharePoint Designer – Todd Baginski
Creating a Custom Composite Activity – Morgan Skinner
I'm still hoping the WF team will one day publish and document the source code to the base activity library in WF. This would be a great starting point for developers who need to build robust, production ready custom activities.