I discovered Vagrant recently, and quickly fell in love as I realised it power and awesome.
It's basically a wrapper over Oracle's VirtualBox (which by it's self is a great tool) but when combined with the Chef/Puppet scripts a whole new world of automation unfolds.
It will take a little time to set up your environment initially, but once it's set up it's set up for life.
Now where to find a small Ubuntu Server 12.04 vagrant box that I can trust? Well, I couldn't find any (not even this site). However I did find a fantastic script by Carl Crafoord that packages up a Vagrant box in a bash script.
The only teeny tiny issue is that it was build for a Mac, so I had to port it onto Ubuntu 12.04 Desktop edition and also changed it to download the ISO via torrent (transmission-cli), which is way, way quicker.
You can get it here: https://github.com/just-digital/vagrant-ubuntu-precise-64 or better:
$ git clone https://github.com/just-digital/vagrant-ubuntu-precise-64.git
I've tested it but only on my local PC. Please let me know if you have any issues.