First, the 'Best Practices for creating Custom Activities' endpoint.tv piece (Part1, Part2).
Second, wf.codeplex.com! (This probably classifies as old news, but there is now a general team site on codeplex wf.codeplex.com. Cool!)So far, it features:
Last, on the workflow unit testing front,
Ron Jacob recently posted some WF4 unit testing helpers he created on CodeGallery.And in case you're not a reader of Kushal's blog, a while ago he had a terse piece about mocking workflows by providing a custom Xaml type resolver which is pretty interesting.