[since the weather radar mosaic has been proven very popular, I felt it deserved it's own announcement]
Part of the sister announcement of the global and regional weather prediction layers, in GeoMet services, there's also the popular weather radar North-American mosaic. It covers United States and Canada in a single layer available in WMS and KML. The data is refreshed every 10 minutes and can be easily animated via the KML file. Here's the link to the WMS GetCapabilities and the KML file for Google Earth. For Esri users, here's the ArcGIS Online map with the layers. Interested in staying in touch with the improvements to GeoMet, join the announce mailing list. (Disclaimer: I am personally involved with GeoMet)
According to the WMS GetCapabilities, the weather radar mosaic is provided as 4 different layers:
From the documentation page: "Meteorological layers are dynamically served through the Web Map Service (WMS) standard to enable end-users to display meteorological data within their own tools and on interactive web maps, and served through KML standard for easy display in tools such as Google Earth™. [...] This service is aimed at specialized users and developers with good meteorological and information technology knowledge who want to visualize raw NWP models and the weather radar mosaic layers in common geospatial tools; want to integrate meteorological layers into their own platforms, or; want to display meteorological layers on interactive web maps for the web or on mobile devices."
Those interested in the U.S.-only weather radar layer in WMS and WCS described as "Mosaic of National Weather Services RADAR for CONUS 1x1 Km resolution" can head on this NOAA website. The following image provided by Environment Canada comes from the GeoMet services but that's not the weather radar mosaic layer.