One of the first questions I'm asked by those who see the Dramble is, "How do I build my own?" Since I've been asked the question many times, I put together a detailed parts list, and maintain it on the Dramble's project wiki on GitHub: Raspberry Pis and Accessories.
For a little over $400, you can have the exact same setup, with six Raspberry Pi 2s, a network switch, a rack inside which you can mount the Pis, microSD cards for storage, a 6-port USB power supply, and all the required cables and storage!
I also include detailed instructions for building a small breakout board for the Raspberry Pi's GPIO with a software-controlled RGB LED, so you can do things like monitor web requests using the RGB LED, show server status, or even use your Raspberry Pi as a flashlight in the dark!
But what if you just want to tinker with Drupal (or other software), or play around with RGB LEDs and other hardware through a Raspberry Pi GPIO, but you don't want to purchase six of them?
I have a separate project, that I use for smaller-scale testing, Drupal Pi. This project aims at configuring a single Raspberry Pi with the LEMP stack (again, using Ansible), and then installing Drupal 8 on the Pi.
I used to also maintain a lightweight Raspbian distribution, Diet Raspbian, specifically tailored for headless Pi servers, but now Raspbian has an official 'lite' version, so use that instead!