First off, Ruby on Rails has always been fascinating to me, but I haven't done any RoR development in a couple of years, so what better way to get a refresher on the material than to spend a weekend working through the Rails tutorial from http://ruby.railstutorial.org? I'm super excited, but as a developer, you want to be efficient with the way you use your time, so it can be bothersome to have to setup a development environment from scratch.
This is where Vagrant comes in. I have only discovered Vagrant recently, but I have come to love it. I can use Vagrant to throw up a dev environment for me in a matter of minutes without much hassle. Then from there I can just configure it for Ruby on Rails development myself OR browse the internet for a configured base RoR Chef cookbook that will set up everything for me. I have used Vagrant for other dev environments too such as NodeJS and PHP. Another great feature of Vagrant is the ability to save your box as a base box that OTHER developers can use. For example, let's say you're working with a few developers on a project and everyone should have a similar base dev environment to start with. With Vagrant, you create a base box and let the other developers use it. It's that simple and you won't have developers making excuses that things work in one environment and not another's.
If you haven't heard of Vagrant yet, check it out at http://www.vagrantup.com/. You will definitely not be disappointed.