Google

• Visit the Google Calendar URL
• Click the settings menu button (the one displaying a cog) on the top-right side of the page
• Click the Settings option
• Find the Settings for my calendars section under the sidebar
• Click the calendar that you wish to export
• Access the   section
• Copy the URL contained under the Secret address in iCal format field

The resulting URL should look like this one:

https://calendar.google.com/calendar/ical/your.mail%40gmail.com/private-[A_LONG_SECRET_KEY]/basic.ics