Server
The server I work on almost every day.
The use cases:
website hoster
Mostly hosted with Nginx, crow cpp, Postgresql/Mariadb and Node.js.
Virtual private network
Openvpn with its own dns server. The dns server is configured as a caching server, meaning that the result of each dns query is saved. This boosts my online privacy.
Email-server
Postfix and Dovecot. This is just the best way to avoid spam.