http://shapewiki.com is a an open repository for shapes you can use on Google Maps. It includes a point-and-click editor for creating new shapes, and is backed by a tagged, searchable database so you can find shapes made by others.
There are three export formats:
- JSON-- essentially an array of latitude/longitude pairs with some additional metadata
Potential future enhancements include: export as encoded polylines, and import from a variety of formats.
When you try out creating a shape, notice the reference image section on the right-hand side. Enter a URL there to pin an image to the map so you can trace neighborhoods, etc. Here's an image of the neighborhoods in San Francisco: http://real-estate-us.com/images/SanFranciscoNeighbourhoodsCA.gif