ArcGIS JSON to GeoJSON
I just released a tool that utilizes Terraformer and Restify to make it easy to convert (and map using GitHub's GeoJSON maps) the JSON you get from ArcGIS Server Endpoints (like this) into GeoJSON. There's even a bookmarklet that gives you a 1-click option.
Tip: Make sure you set the Out Spatial Reference when doing your query on the ArcGIS Server to 4326 to get the best chance of success.
Here are the pertinent links: