One of my colleagues has consulted me what is the best way to save common configuration for web parts provided that this configuration can differ from site to another.
And my answer was create a custom list in each site has two columns key and value, the administrator can set the values and the web parts in the site read from it.
After while I found this great blog post Introducing the SharePoint Config Store for developers which introduced the SharePoint config store which this functionality in a neat and clean way.
This the description from project site on codeplex
Project Description
The SharePoint Config Store is intended for SharePoint developers, and provides the framework to be able to use a SharePoint list for application configuration values. This is useful when there are certain values used in a site or application's code, but we don't to hardcode them or even store them in web.config. Storing such values in a list means they can be easily updated (possibly by end users if you choose to allow this) without requiring access to the web server's filesystem.