If you would like to create your own personal map with just the features you want, check out:

• openstreetmap.org
• openlayers.org

For even more fun, build yourself a database of geocoded addresses from the 2010 US Census with Post Gis 2.0

Rocket Science for the masses--how great is that?  Suppose you need to know for sure if your customers are within a certain municipal boundary.  Boundaries such as these aren't described in straight lines and tables, you'll need software that can read polygons.   That is a little of what Post Gis does.  Post GIS allows a database programmer who usually deals with 2 dimensional tables be able to query irreguarly shaped objects and make tables!