WebView screen saver, and it's free and open source

Release 1.0.0 bundle and sources are here: https://github.com/okertanov/Icoon

To install just unzip and right click then "Open" (sorry, apps based on osx plugin architecture can't be deployed via AppStore so it warns about unsigned app).

Now you can add http://tweetping.net/ or http://www.google.com/trends/hottrends/visualize or http://www.donothingfor2minutes.com/ or any of the other sites like build status, or monitoring tool.

And if you know such zen-like/fullscreen-alike pages please post them here too and I'll push the README.md update.