• Useful Sharepoint Designer Custom Workflow Activities - This project takes aim at making it easier to create advanced workflows using Sharepoint Designer by providing a set of custom workflow activities.
• SharePoint Designer Workflow Extensions (custom actions - Provides the ability to create generic C# (and potentially other managed .NET) code implemented as an extension to SharePoint Designer.
• Iron Python Activity - adds an IronPython script activity to SharePoint Designer allowing busy developers to manipulate their workflows with ease

I'll keep updating this list as I find more...