This process is now simplified (July 2014) compared to the steps I see in the accepted answer. It now seems much easier to get this data. I at first floundered around the web hoping I could just download a bunch of standard maps in GeoJSON format, but came up empty other than standard US/Canada offerings. As of right now there doesn’t seem to be a lot available in straight GeoJSON. Instead you take an older, widely used format to generate GeoJSON. This is easy, and a good path to take. We’ll be working with shape files and converting them to GeoJSON.
-
First download a shape file for the geographic area you are interested in. A shapefile is a digital vector storage format for storing geometric location and associated attribute information. (http://en.wikipedia.org/wiki/Shapefile)
There are lots of sources of these. These are sources I found useful:
GADM – Download data by country or one giant file for the world. Each zip you download has multiple shape files inside starting at number 0 and increasing. The higher the number the higher the detail level. Like country, state, county, etc. (http://www.gadm.org/country)
Another download site (http://www.naturalearthdata.com/downloads/)
Download US State, County, Sub-County data as driven by the census bureau – http://census.ire.org/data/bulkdata.html
-
Once you have your shape file, drag and drop it into the webpage at http://www.mapshaper.org. Here you can drag a slider to change the vector resolution. My experience was that 10% resolution looked great still for web maps, 25% was near perfect. File size was greatly reduced, so I would recommend using it. My Massachusetts map went from 800kb of GeoJSON data to 80kb after reducing resolution.
-
Click the GeoJSON button on mapshaper and the file is automatically exported for you.
Optional – Once you have a shape file, you can edit it for free in a tool like Quantum GIS (QGIS).
You can also hand map GeoJSON data at this website. http://geojson.io/#map=2/20.0/0.0