If you run game servers, host things for other people, or just want a clean way to manage multiple server processes on a machine, Pterodactyl is probably the best free option out there. I've been using it for years and it's the backbone of how the server host I work at works.
What it actually is
Pterodactyl is an open source game server management panel. It gives you a web interface where you can create, manage, and monitor server instances — each one running in its own Docker container, isolated from everything else. You get a file manager, a console, resource limits, user management, and a full API. All for free.
How it's structured
There are two main components. The Panel is the web frontend — that's what you and your users interact with. Wings is the daemon that runs on the actual machine and manages the containers. They talk to each other over an API. You can have one Panel managing multiple Wings nodes on different machines, which is how hosting companies scale.
Eggs
Eggs are configuration templates that tell Pterodactyl how to set up and run a specific type of server. There are community eggs for pretty much everything — Minecraft, Rust, CS2, NodeJS apps, Python scripts, static sites. If there isn't one for what you need, you can write your own, which is something I've done plenty of times for the server host I work at.
Why I like it
Mostly the control. Every server gets its own resource limits so one greedy process can't eat the whole machine. The API means you can automate basically everything. And because it's self-hosted, you're not paying per-seat or dealing with someone else's uptime problems.
It's also just well built. The codebase is clean, the documentation is decent, and the community is active. For something that's completely free it's genuinely impressive.