Every time my son gets invited to a friend's house or a party, or if I even just pick up someone to take with us somewhere, I go to Google Maps, print up a copy of a map to their house, write the child's name, phone number, address, and parent's names on the printout. I then fold it up and paperclip it with the group of others in the glove compartment of my car. That way, it is always there when I need it at the last minute (which is when my son plans most of his outings). It is even there for "emergency" situations when I need to quickly call a parent if I'm late.
Those with nifty GPS units can save friends' addresses in the unit's address book. Web-connected phones and mobile devices can make the Google Maps connection straight from their contact listings.