GCal Popup is a Firefox extension for Google Calendar users too lazy to open a new tab when they want to enter a new appointment or check their calendar. Once the plugin is installed, you'll see a little calendar icon in your status bar. Click it and your Google Calendar will pop up, over the top of whatever page you happen to be visiting. Click the close button and it will go away.
If you're already logged into your Google account you won't even need to login to Google Calendar when the window pops up.
This is listed as an experimental plugin, which means you'll need to register for a Mozilla account and login before downloading. But the add-on seems to be stable enough and hasn't caused my computer to burst into flames. Yet.