Yes there is NoSugar!
If you are using Outlook 2003 utilizing pop3/smtp it is easy as pie. When you create a new calender event or modify an existing one there is a field to add attendees.
Enter the person whos calendar you want this to appear on. They will be automatically sent an invitation for the event. Once they accept it – it is added to their calendar. Any changes you make will be sent to them as well.