Docker (previously dotCloud) made a big splash this year when they open-sourced their software for creating "lightweight, portable, self-sufficient containers" that powers their Platform-As-A-Service offering.
Developers are excited because Docker offers an easier to use alternative to Chef and Puppet for managing server environments. Instead of wrangling with configuration files, Docker allows developers to simply take an image of their system and share it with their team. When a team member makes a change to their local environment, they just create a new image (a Docker container) and share it with the team. Its like git for disk images.