If you've ever build SharePoint Workflow's with SharePoint Designer, you've probably developed a "Love/Hate" relationship with the product. One of the best things about the tool is that it allows a non-developer (although having a development background is a plus!) to build SharePoint/MOSS 2007 workflow's without having to use Visual Studio. The SharePoint Designer team has done a great job in adding a bunch of "Out of the box" or OTB workflow activities and functionality that mimic, more or less, what you might use/build in Visual Studio. However, these Out of the box activities are limited in quantity and in some cases lack in the functionality that you might want. If you are, or have access to, a Visual Studio developer you could have the best of both worlds: The RAD/User-centric way of building SharePoint Workflow with SharePoint Designer with the custom-built features of Visual Studio-based workflow, by "importing" the custom built activities/workflow's from Visual Studio into SharePoint Designer. But how?
Well, I just found a great post that shows you how to add Custom Workflow Activities to SharePoint Designer, written by a Microsoft MVP (Carlos Sequra Sanz, his blog has posts in Spanish and English, and you can find it here). Carlos shows step by step how to do this (in English). I haven't had a chance to run through the code myself, but on first look, it appears to be correct and well worth the 20 minutes or so you might have to invest to read what he wrote.
He did it in a two part post, linked below: (Both are in English)
Great job Carlos!