Microsoft's Patterns & Practices - Enterprise Library
Enterprise Library - homepage on MSDN. Additional code and documentation available on the project page at Codeplex.
The latest release is 4.1, dated October 2008. According to the documentation there is no Configuration Block anymore. I find that with version 2.0 the Configuration Application Block was replaced with IConfigurationSource. Apparently this block only made things worse and was thrown out. Well, I remember the first time and I was hoping things have improved by then but I was wrong. They, actually, did the right thing and eliminated the whole block out of Enterprise Library.
So, back to creating my own configuration solution. :)