Google Mashups is the term used to describe facilities offered by Google which can be incorporated by anyone to create new software products.
It provides a general mapping facility and you can import your own data to display on a map of any area in the world.
There is a range of free data sources which you can use for this purpose. For instance, in the UK, the Highways Agency.
And if you are in the UK, where the Government-controlled OS charges exorbitant fees for access to data paid for by taxes,
then you will be interested in the Guardian's 'Free Our Data' Campaign.
This gives valuable publicity to a number of initiatives aimed at providing copyright-free alternatives to Government-controlled data.
For example, OpenStreetMap.org.