Some time ago i would have added OpenVZ to the list, which is a para-virtualisation solution (basically a sort-of chroot environment, quite like a Solaris zone), but i as far as i know the future of it is (at least for an outsider like me) unclear at the moment. One of the sites i know of that are using it plan to switch over to Docker or may have already done so.

As far as i have seen it in action it looked like a rather mature and good solution and it was very ressource-friendly too.

I hope this helps.

bakunin