Mailbox Map is a Google Maps mashup showing the locations of mailboxes in the USA.
Search for an address or zip code and Mailbox Map will display the locations of mailboxes in that area. If you click on a mailbox map marker the map will display the box's collection times.
The site also has a mobile phone version of the map that displays USPS mailboxes, US post offices and UPS stores.
UK Postbox Finder
The UK Postbox Finder can help you find a Royal Mail postbox in the UK. To search the map just enter your postcode and a map will display your nearest postboxes. The postboxes are also listed in the map sidebar with the time of the last collection.
The site uses the Google Maps API with OpenStreetMap map tiles.
USPS Mailbox Locator
The Mailbox Locator helps you find USPS collection stations (blue mailboxes) and post offices in your area, and also offers the opportunity to discuss the status of individual blue mailboxes.