I run a few servers that I use for experimentation and learning. Currently, my lab consists of an HP Z420, Dell Optiplex 780, Raspberry Pi 2, ODROID-XU4, Rock64 and RockPro64.
These systems are not traditionally found in a homelab, but they allow me to experiment with many technologies at a low cost, both upfront and in terms of power. The ARM systems (Raspberry Pi, ODROID and Rock64) are especially good for this.
All these systems run NixOS, an innovative Linux distribution, with other OSes running in containers and VMs.
Some of the services and technologies currently in use in my lab are Docker, NGINX, ZFS, Flask, KVM/libvirt, InfluxDB, Telegraf, Buildbot (building Arch Linux AUR packages), OpenVPN, Ansible, BIND9 (DNS server), Syncthing, and Sanoid.