Vagrant is a Ruby-based tool for building and deploying virtualized development environments. It uses Oracle's open-source VirtualBox virtualization system along with the Chef configuration management engine along with lots of Ruby goodness to automate the creation and provisioning of virtual machines for development purposes.
If you thought rolling out new VMs using Amazon EC2 was easy, Vagrant brings an even simpler system to your local development machine. From the command line, starting is as easy as:
sudo gem install vagrant
vagrant box add base http://files.vagrantup.com/base.box
mkdir vagrant
vagrant init
vagrant upBe warned, though - as a 370MB download, adding that box image isn't a quick process! Once you've got it though, you can keep rolling out VMs based on it at will.
Note that VirtualBox is a separate dependency for Vagrant. Vagrant does not come with VirtualBox built in or anything like that. Download VirtualBox for your OS if you want to try Vagrant.
Beyond the basics of getting a VM running, Vagrant can take care of port forwarding, distribution, environment setup, SSH access, shared folders and, importantly, the provisioning of software onto the VM using Chef. If you want to automatically roll out a VM with Apache 2, Rails, Phusion Passenger, or the like, Chef and Vagrant will take care of it for you. This is powerful stuff!
In terms of documentation and having a straight forward official homepage, Vagrant sets a solid benchmark. There's a straightforward guide to getting started with Vagrant, lots of documentation, and a 12 minute getting started video/screencast. Great work guys!