[CentOS] [OT] Conference room software

We have a very decentralized user base with no standard operating system or utilities except for a web browser of some kind (no Exchange either). What recommendations does the list have for software to coordinate conference room reservations? It doesn't need to be responsible for any kind of invitations or email notifications, really just a pretty calendar where scheduled events can go. It will first be used for reserving conference rooms but eventually could be used for more.

Basically, a web application that can run on whatever web container installs on linux which supports shared calendars. Google calendar behind the firewall :) ... any suggestions? Thanks.