Thanks to Chris Lee for adding me to the Planet!
First, an introduction. Hello, I'm Shawn Starr, I've been busily working on the weather engine for Plasma. The plan is to begin working on an applet as soon as some javascript bindings for the dataengine are ready to use.
Currently, You can get weather data from Environment Canada (conditions + 5 day forecast and more) and NOAA's National Weather Service (conditions only). Work is underway with the BBC/UK MET Office ion.
You may have seen aseigo's screencast demoing the weather dataengine.