ICal
Originally released as a free download for Mac OS X v10.2 on September 10 2002, with the release of Mac OS X v10.3 it was bundled with the operating system as iCal 1.5.  Version 2 of iCal was released as part of Mac OS X v10.4, Version 3 with Mac OS X v10.5 and Version 4 as part of Mac OS X v10.6.