SharePoint Designer (SPD) is a great Rapid Application Development (RAD) tool for building workflow (Document and other) for the Microsoft Office SharePoint Server 2007 product.  It is a tool that both developers and non-developers can use, albeit at a different level of capability.  That being said, you can get a lot done without writing code with this tool.  If you are a Visual Studio Developer you will find it a great tool to quickly start building a Workflow, and since it uses the same Workflow Engine as Visual Studio (i.e., Workflow Foundation), you can open the workflow's build in SharePoint Designer and modify/enhance them in Visual Studio 2005 and vice-versa.

Here's the product page (here) and this is where you can download a free trial version (here).

If you want to see it in action, check out my recorded Webcast in which I use the product to build a custom document workflow here.

~ Robert Shelton, Jr.