What is Vagrant? It's a neat command line interface and toolchain at top of VirtualBox, which allows you to create and manage virtual "boxes" - vm instances with stuff configured for your project.
Why is it awesome?
Because you don't have to setup everything for the project every time when hop in, you do it once using one of the base boxes, you package it and populate *.box file across all project members. When you need to add some software (new database, cache soft, etc), you just have to add it once, and ping everyone to update their boxes. Briliant, isn't it?
It works great also with designers and non-technical people, download and setup of a box takes few minutes and they don't have to polute their machines with all the project related software.
Above all, no more excuses "works for me", or "works on development, but not on the production" - as boxes are made to simulate production environment as much as possible.