MapProxy is an open source proxy for geospatial data. It caches, accelerates and transforms data from existing map services and serves any desktop or web GIS client.
MapProxy works like a simple tile cache, but also offers many new and innovative features like full support for WMS clients.
MapProxy is actively developed and supported by Omniscale, it is released under the GNU AGPL License 3.0, runs on Unix/Linux and Windows and is easy to install and to configure.
Go to http://mapproxy.org/ for more information.
The documentation is available at: http://mapproxy.org/docs/latest/
Note: The configuration format changed between 0.8.x and 0.9.x, so you have to make sure you configuration matches the new format if you're going to upgrade.