Citadel

Citadel provides a self‑hosted suite of groupware tools that include email, calendar and scheduling, address books, forums, mailing lists, instant messaging, wiki, blog, and RSS aggregation. It is built around a “rooms” architecture that presents these services through a consistent interface, allowing users to manage communication and content without switching between disparate applications. The system supports multiple domains and can be installed on personal hardware or in cloud environments, offering a full‑featured collaboration platform that runs in minutes.

The software is written in C, emphasizing lean performance and memory safety, and it is released under the GPL‑3.0 license. All components are available without subscription or tiered restrictions, and the project maintains a stable, mature codebase. A web interface provides administrative control, while the community offers documentation, FAQs, and support resources.

Citadel targets organizations and individuals who need an integrated, open‑source alternative to commercial groupware, offering email, scheduling, messaging, and content management in a single, extensible package.