Citadel has a nice calendar service with AJAX-style web access, along with WebDAV (WebCal) access. It is a true turnkey groupware environment — very easy to install. It comes with not only a calendar service, but also email, contacts, instant messaging, bulletin boards, and more.

If you’re looking for an open source “Exchange killer” that’s incredibly easy to install, use, and maintain, check it out. The project lives at http://www.citadel.org . There’s even a pre-built VMware image of the system that you can use to evaluate it.

Citadel is a complete and feature-rich open source groupware platform.

• Email, calendaring/scheduling, address books
• Bulletin boards, mailing list server, instant messaging
• Multiple domain support
• An intuitive, attractive AJAX-style web interface

Users love Citadel because it’s software that helps them work, play, stay in touch… without calling attention to itself. System administrators love Citadel because it installs in minutes without the need to manually integrate all the different components together.The key to Citadel’s versatility is its unique architecture. A Citadel system is made up of containers called “rooms.” A room may be used as an email folder, a discussion forum, a real-time chat, a mailing list, a calendar, an address book, an RSS sink … sometimes a combination of any of the above, and certainly any other uses which could be added in the future. Furthermore, you can replicate rooms between multiple Citadel nodes, allowing you to set up a federated, distributed messaging environment.