Internet Calendaring and Scheduling Core Object Specification - relevant for both sharing (Group Calendaring) and Data Synch
http://www.ietf.org/rfc/rfc2445.txt
XCal is the XML DTD spec related to it.
Some comparisons http://wmf.editthispage.com/discuss/msgReader$7900?mode=day to VCal
Apparently http://wmf.editthispage.com/discuss/msgReader$7904?mode=day Ms Outlook (and Ms Entourage) can work with ICal format.
pointers http://farm.tucows.com/blog/_archives/2004/11/2/173619.html
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Also Apple's Mac calendar software/service.
There is some Group Calendaring potential, but it's not clear whether it's compatible with anything else.
http://www.apple.com/ical/share.html
http://apple.slashdot.org/apple/02/09/13/1843217.shtml?tid=177
http://www.macdevcenter.com/pub/a/mac/2002/09/20/ical_webdav.html