The best workflow tool for SharePoint - PERIOD.
This is an amazing package. It allows completely code-free control over SharePoint workflow. If you have any experience using the Visual Studio interface to program SharePoint, you will appreciate just how complex SharePoint workflow coding can be, and Nintex Workflow for SharePoint eliminates this complexity. The product itself can appear quite complex at first, because it has a huge array of elements available to define custom workflows. But once you've been introduced to the basics, it's really quite straightforward. The product is elegantly designed and deeply integrated with SharePoint. My only reservation is with the pricing - licensing can be expensive. But the value is substantial.
Simple, graphical interface for workflow definition
Licensing can be expensive