Configuration Management Application Block
There are several Application Blocks from Microsoft lately, but the Configuration Management Application Block (released 6/24/2003) is one I have been keenly interested in since I started working with configuration files in .Net (and other projects before .Net). I have spent some time in other projects working on interfaces to seamlessly manage configuration files (reading/writing) using encyrption, etc.
One item I felt was lacking in 1.0 was the easy integration of encryption in XML configuration files (in particular, the web.config file and machine.config file). You could do it, but you needed to go through several steps to accomplish it. With 1.1, this is more or less done natively now, but it would still be nice to encapsulate some of the logic. The Configuration Management Block provides this for you and more.
The other great feature the Configuration Management Block provides is seamless integration with configuration sources other than XML files. You could use the registry, a database, or many other storage mediums, all using a common interface to manipulate the configuration information.
Also, as I mentioned here in comments, it is only available for the 1.1 .Net Framework (the first of the Blocks, I believe, that it is NOT allowed to install on a 1.0 .Net only machine).
Take a look!