I haven't spent too much time investigating this as we've still been pretty busy about planning our SPS-CMS implementation, but I do plan to dig into it. I have seen it get lots of good press in numerous blogs.
An extension framework for building SharePoint Web Parts. The initial goals of this framework is to extend the base Web Part class with features needed in most common Web Parts. First and foremost, we have integrated the log4net Logging Services to facilitate the consistent logging of exceptions and program flow. As the framework grows, we will provide more documentation and examples. If you use the framework tell us and the community about it. Share your knowledge and code!