How does Linux replace MS Exchange's calendaring and scheduling features?
JiCal is one answer to that perennial question:
How does Linux replace MS Exchanges calendaring and scheduling features?
The JiCal answer is:
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.