There's a variety of road condition sites managed by the DOT's from the US and Canadian governments. The link to the Federal directory site is here.
National Weather Service: Quick & easy access to all U.S. weather warnings & watches, temperature forecasts, flood stage alerts, infrared & satellite imagery. The site also has a gallery of awesome photos of severe weather.
This thread will be maintained as a Sticky for those road condition sites that we deem especially helpful.
(This is map of the region and is an especially good use of the technology). Another link for Arizona is this "High Impact Traffic Alerts."