Here are step by step directions to subscribe to a public Google Calendar.

If you are the calendar owner
Be sure the calendar is shared.  
Go to Calendar Settings > Share this Calendar and set the check box to Make this Calendar Public.

Calendar Settings

Calendar Settings > Share this Calendar

Now go to Calendar Settings > Calendar Details and get the URLs to share with friends.

Click on ICAL and copy the URL that appear in the popup window.

If you friend uses Google Calendar, they will need the ICAL url. iCal is a calendar format developed by Apple and used by many companies as a defacto industry standard.

Subscribe to a Shared Calendar
If you use Google Calendar or a compatible calendar software and your friend just gave you an ICAL URL, you can subscribe to their calendar.  Here are the steps using Google Calendar.

Remember: This is a subscription, not a copy.  If changes are made on the original calendar, the updates will appear on in your Calendar. You will not have permission to change your friend's calendar.

1. Log in to GMail and click Calendar.

2. Click Other Calendars > Add by URL


3. In the pop-up window, paste in the ICAL url that you got from your friend.

4. The calendar should now be listed under Other Calendars.