We have been religiously using Google Calendar to get SMS alerts for important events, using it as Desktop Calendar, synching events with Outlook and back and getting Google calendar event alerts in Firefox.
However the one missing feature in Google Calendar was the ability to access the events offline through a browser, but seems like Google has finally made offline access to Google Calendar available to all users.
To activate the offline feature, click on the Offline beta link on the top of the page. However the features in this version is very limited and you can only browse already created events in offline mode, making it a read-only option, but you will definitely see more option in the coming future.
Note: You will have to download Google Gears first to get offline access to Google Calendar.