Ruby on Rails

Vagrant is really nice, but I see it more as a solution to test your production environment locally than to manage gems or ruby versions.

Those VM images can be quite big, but are great to use as a staging server to make sure your production server will be able to run your Rails app (or an update) without any problems. Setting up your production environment on a virtual machine (a staging server if you want to call it that way) is great, especially combined with Chef. You can be fairly certain that your production server won't go down because of some incompatibility or different/lacking dependencies between your e.g. OS X machine and your Ubuntu production server.