Neighborhood API provides boundary data defining neighborhood throughout United States. It acts like a location twitter feed, and you can post to neighborhoods and request a neighborhood stream. It also provides a simple Google Maps implementation, which You can see in the map to display and interact with neighborhood boundaries.
Anyone who lives in even a smaller city knows the importance of neighborhoods. Portland (where I live), for example, is really more like a collection of many connected small towns. Each has its own handful of coffee shops and restaurants. The people behind GeoReach API (our GeoReach API profile) understand this. The API provides neighborhood boundaries, along with local content.