Citadel

Citadel is a self-hosted all-in-one platform that gives you a website builder, groupware, email, instant messaging, and calendar tools. It is designed to be run on your own server, keeping your data under your control. The website builder module lets you create and manage web pages without needing to write code, making it useful for small businesses, clubs, or personal sites.

The platform runs on Linux and includes a built-in mail server (SMTP, IMAP, POP3), a webmail client, a chat system (group and private messaging), and a Caldav-compatible calendar. Citadel has been in development for over two decades, originally starting as a BBS system. It is written in C and uses its own database engine instead of relying on MySQL or PostgreSQL. The web interface is accessible from any modern browser.

Citadel is free and open source software released under the GNU GPL. It is best suited for administrators who want to run their own server rather than use a hosted service. The project maintains detailed installation guides and an active community forum. Because it packs many services into one package, it can simplify server management for small organizations.