JiCal is an answer
Check this out: JiCal... it looks interesting:
Use Ximian Evolution or Mozilla Calendar on your Linux desktop and JiCal to publish free/busy time to your webserver. this will give you scheduling capabilities similar to Exchange Server but using open standards based on rfc2445.
It also works with Outlook. Very nice - a friend of mine and I were trying to come up with something similar a while ago and we called it EventEngine... but like all my grand ideas, it got shuffled off to the side.
-Russ