Besides Zillow's creative-commons licensed neighborhood boundaries shapefiles, what sources of neighborhood boundaries are there? Not just in the US, but all over the world.
Free/open source is ideal, but paid sources could be interesting too.
SimpleGeo (before being acquired by Urban Airship) used to have a worldwide neighborhood boundaries layer. If you do some digging you may be able to troll up an old shapefile, or maybe a kind sole out there already has a copy that they would be willing to let you download.
the research effort by some keen Carnegie Mellon U mobile lab students (http://mcom.cs.cmu.edu/) analyzed 18 million Foursquare check-ins to spot algorithmic relationships between the spots people frequent. According to the work… “Livehoods looks at the geographic distance between venues, but also a form of `social distance’ that measures the degree of overlap in the people that check-in to them”.
Check out commercial data provider http://www.maponics.com for neighbourhood boundary data.
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As the boundaries of neighborhoods are often nebulously defined I think no matter the source you will have to understand that using them for anything more than general display/labeling is going to be tricky.
How about the Flickr Shapefiles which are constructed from the location tags people give geolocated pictures on the site.
Maponics can supply you with neighborhood boundaries . If nationwide drops of shape files are too large or beyond your budget, you can also use our API that enables you to download just what you need by attribute, geographic window or ID.
In addition, we also have "Context" data that is associated with our neighborhoods, carrier routes and other boundaries that can tell you all sorts of things about these polygons including things like Clarion Prizm demographic data!
Disclaimer: I work for Maponics
For Australia I would be looking for 'Locality' Boundaries - these are 'suburbs' and are usually legally defined. The term 'Neighbourhood' is used in Australia but there are not usually legally defined boundary areas for 'Neighbourhood' whereas 'Localities' are legally defined and mapped by the Land Authorities in each state. Try: http://data.gov.au/dataset/bdcf5b09-89bc-47ec-9281-6b8e9ee147aa/resource/d2c450e6-5ad3-45fc-a1da-b82cd30f2c47/download/suburbs---localities-may-2017.zip
Many world cities publish data that belong to the public on open data pages. These usually include only official boundaries for city planning, budgets, and the like. Peter-Moeykens is correct that Maponics has much more detailed neighborhood polygons with names that change from year to year. The best data is proprietary, but public data can be quite useful.
For cities outside the English speaking world, you might need to search with the right translation for "neighborhood." For example, the Mayor's Office of the City of Paris has ParisData. By searching for "quartier" (neighborhood) you'll quickly find the official boundary data available in several formats. Take a look at the "carte" tab to preview the geometry. http://opendata.paris.fr/explore/dataset/quartier_paris/map/?location=12,48.85889,2.34692
The text description doesn't machine translate very well: Données géographiques surfaciques. Le quartier désigne la division administrative de l’arrondissement. Chaque arrondissement est découpé en quatre quartiers administratifs. Paris compte ainsi quatre-vingt quartiers administratifs.
Geospatial data. 'Quartier' (neighborhood) refers to the administrative division within the 'arrondissement' (city district). Each 'arrondissement' (district) is divided into four smaller administrative districts. Paris thus totals 80 administrative 'quartiers'.
See here for images that explain the 20 arrondissements and 80 smaller divisions: https://en.wikipedia.org/wiki/Quarters_of_Paris#Chart_of_the_eighty_quartiers_of_Paris