Source for GeoJSON data for the US

I found something closer to what I was looking for (US State and County boundaries) on github: https://github.com/johan/world.geo.json/tree/master/countries/USA

The features are very generalized/simplified, but they'll work for a national scale map. Country boundaries for the whole world are available in this repo too.

After trying to find similar GeoJSON versions of US State and County boundaries, I finally decided to roll my own from the United States Census Cartographic Boundary Files, converting them to GeoJSON and KML formats using the MyGeoData vector converter.

I've made the results of this conversion available at: http://eric.clst.org/Stuff/USGeoJSON. Feel free to use these if they will help.

You might take a look at SimpleGeo’s CC0 Places data set, and this related question: Converting SimpleGeo Places GeoJSON file to shapefile?

or just searching the geojson tag may turn up some results for you.