If you're running linux you can use the lxc plugin for Vagrant[1], I'm on the devops side of thing so end up needing to spin up a lot of machines from time to time to test clustering etc... and the lxc plugin has been a saviour as you can get much higher density of "machines".
The only caveat is you can't do things modify kernel params etc... but that's usually ok for my use YMMV obviously. Also they don't update Atlas with new images so you need to build your own but that's really easy[2].
That sounds interesting, I generally deploy to vagrant via ansible using the same config as production (just a different file with configuration variables) since that allows as close as possible replication of the deploy environment (particularly for older projects on 14.04 when I'm running 16.04 on the dev machine), lxc would be useful for those "I need to simulate lots of machines" configurations though.
I wasn't aware that it had gotten as good as those links look so thanks! :).
The only caveat is you can't do things modify kernel params etc... but that's usually ok for my use YMMV obviously. Also they don't update Atlas with new images so you need to build your own but that's really easy[2].
1. https://github.com/fgrehm/vagrant-lxc 2. https://github.com/obnoxxx/vagrant-lxc-base-boxes