If you haven’t yet heard of the Chandler project now is the time to change that. You can read more about the calendar system here: http://chandlerproject.org/.
You can take a full 3 minute product tour by visiting this page: http://chandlerproject.org/Projects/ProductTour.
More than just being compatible with iCal which came with the MacOS X, it can actually be used by people using either Windows, Linux or MacOS. True cross-platform programming.
For developers and businesses who want to take this Open Source programme up a notch full source code can be collected at http://chandlerproject.org/Developers/ChandlerDesktopSource so Chandler can become your ultimate calendar in businesses, and for private users you get a Personal Information Manager with quite a bit of potential – for free. Could it get any better than this?… 
Update on August 21th 2008 – my into to why YOU need Chandler:
Chandler helps with a tight schedule – http://tinyurl.com/5zma4r on 12seconds.tv