Odroid Cluster
I use a cluster of 5 Odroid-XU4s(https://www.hardkernel.com/shop/odroid-xu4-special-price/) for hacking around with distributed systems.
That they are 32 bit arm machines makes it interesting. It's hard to find official builds for a lot of software so the provisioning isn't as straightforward as it would be on Raspberry Pis.