Quality Open Source Calendaring/Scheduling?

Jim R. Wilson writes: "In past jobs, I've used Microsoft Outlook/Exchange, Novell Groupwise, and Google Calendar for handling business appointments. I'm sorry to say it, but I have yet to see a rival to Microsoft's scheduling features. On Slashdot, I have occasionally read rumblings that there are better, open source email and calendaring solutions out there. Can anyone substantiate this claim? What are the OSS alternatives? Can any compete with MS's resource scheduling?"