Switchboard
A feature flipper library for Pyramid and Pylons apps.
...Originally used to selectively roll out changes to the SourceForge site, the library lets you easily control whether a particular change (a switch) is active.
You can make switches active for a certain percentage of visitors, all visitors to a particular host in a cluster, or if a particular string is present in the query string. Furthermore you can easily create your own conditions to do fancier things like geo-targeting, specific users, etc. In short, Switchboard turns you into a continuous deployment ninja.