The Google Calendar Quick Add Firefox extension enables a keyboard shortcut in Firefox that adds events to Google Calendar without having to visit the calendar.
Once the extension is installed, hit Control+; (semi-colon) to get a prompt as pictured. Quick add your event and in it goes to your Google Calendar. Keep in mind you have to have logged into your calendar at some point in your session; this baby only works if all your cookies are in order. Free download, works wherever Firefox does.