Switchboard

Switchboard is a port of Gargoyle, a feature flipper for Django apps, to the Pyramid or Pylons stack (including Turbogears). 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.