Unstable releases of QGIS from the 1.x.x series. These releases are more bleeding edge and while generally they should work well, they dont receive the same amount of quality assurance as our stable releases do.
deb http://ppa.launchpad.net/qgis/unstable/ubuntu YOUR_UBUNTU_VERSION_HERE main
deb-src http://ppa.launchpad.net/qgis/unstable/ubuntu YOUR_UBUNTU_VERSION_HERE main
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You can easily enable this PPA by the following command:
sudo add-apt-repository ppa:qgis/unstable