iCalendar is implemented/supported by a large number of products, including:

37signals Backpack
30 Boxes
Apple's iCal application
BlackBerry, Internet edition v2.3 and up
Darwin Calendar Server
Contactizer and iPod
Buni Meldware Communication Suite
Chandler
Drupal with its event module
TYPO3 via its Calendar Base Extension
Citadel
Facebook
FirstClass
Google Calendar
Horde
Jalios JCMS
Joyent Connector
KOrganizer
Lotus Notes
Microsoft Entourage
Microsoft Exchange
Microsoft Works Calendar
Microsoft Outlook (2000 and 2007, but 2003 requires a plugin).
Mozilla Calendar (including Mozilla Sunbird and Lightning)
Mulberry
Novell Evolution
Novell GroupWise
Nuvvo
Simple Groupware
SOHO Organizer
Sun Java Calendar Server
Upcoming.org
Windows Calendar
Webical
Zimbra Collaboration Suite