A cool project on GitHub, which harnesses the power of the hot new application container engine, Docker, claims that it's the smallest PaaS implementation yet. The project is aptly named "Dokku" and it's less than 100 lines of bash code.
Description from the blog introduction:
Dokku is a mini-Heroku powered by Docker written in less than 100 lines of Bash. Once it’s set up on a host, you can push Heroku-compatible applications to it via Git. They’ll build using Heroku buildpacks and then run in isolated containers. The end result is your own, single-host version of Heroku.
– Jeff Lindsay
And here's a sweet video of Dokku in action!
Here are all the components Dokku uses: