Google recently released it’s Calendar Data API’s for developers. With the API’s (Java and C# are available) you can create applications which can interract dynamically with the Google Calendar.
One type of program that I can think of; is one to synchronize Google
Calendar with your handheld’s calendar/organizer (be it a cellphone or a PDA).
Google’s own Description of the Service:
Google Calendar is a free calendar application, and the Google Calendar API Program is an XML-based feed offered for use with Google Calendar (collectively, the “Service”). You understand and agree that the Service may include content-targeted ads or other related information
Allowable free usage is when it is for Personal or Internal Business use only. If it is for commercial use, you must have an agreement with or have the permission of Google.
Sourced from reddit