Dominik Jendroszczyk passed along a tip about a tool that's been released to help make transitioning from the proprietary Google Analytics systems over to the Piwik open source analytics option simple - Google2Piwik.

More and more users are turning away from Google Analytics (GA) as a provider of web measurement solutions on security, independence and functionality grounds. One problem they run into is that after years of using Google’s product they have a considerable inventory of data about their websites stuck with GA, which makes starting over with an alternative provider something of a hassle. [...] Effectively, thanks to Google2Piwik, nothing is stopping you from moving away from Google Analytics to a safer, more robust and more functional alternative.

The tool is written in Python, so you'll need to have that and both access to the Google API and have a Piwik install set up (it's PHP/MySQL based) to make the translation. There's a few limitations to the import, though, so be sure to check out the tool's project page for those. Piwik is an open source project that provides real-time web analytics that installs on your own server and gathers data via a Javascript tag embedded in your site.