I concur, weatherspark is awesome. My only complaint is that the documentation is a little lacking for some of the advanced charts.
Does anyone know if NWS has a good API?
There are number of different ways to access the NWS data and graphs - not surprisingly it is hard to discover them! They do have an XML web service for daily and hourly forecasts http://graphical.weather.gov/xml/ but you can also download binary files that contain all the same weather elements for the US.
I'm actually close to completing a python library that will be open source and provide an easy way to access the NWS national digital forecast database.
It's pretty good. Last time I looked (3-4 years ago), the hard part was getting decent radar data/images.