Docker is perhaps the most popular open source tool that has been released in the last year. The following quote can be seen on the Docker website:
Docker is an open platform for developers and sysadmins to build, ship, and run distributed applications, whether on laptops, data center VMs, or the cloud.
Increasingly, more and more individuals and companies are adopting Docker. The tagline for Docker is Containerization is the new virtualization. At a high level, all Docker allows you to do is prepare lightweight containers using instructions from a Dockerfile and run the container. The same container can be shared or shipped across environments, thereby making sure you run the exact same image and reducing the chance of errors. The Docker image that you build is cached by default; thus, the next time you have similar instructions, the time taken to bring up a similar container is reduced to almost nothing.