Google just released an update to its Maps app for Android, providing offline access to whatever maps you may want on hand.
Google's offline access supports over 150 countries, and can store up to six large metro areas (like greater London, Paris, or New York City and surrounding area) on your device for access when you don't have a connection. It's great for those times you're stuck in an area with no service and need to navigate home, or if you're travelling overseas and don't have an international data connection.
GPS will still work without data, since it's a separate connection, and you can navigate yourself anywhere within that saved area. The new version of Maps also has a smoother compass mode in Street View, for devices that have the feature available.
Maps is a free download for Android devices.
Go Offline with Google Maps for Android [Google LatLong Blog]