Layers Library
Layers are explained in the manual, read here.
Some examples to start the library and the discussion.
Noaa buoy position layer based on a script that parsed the NOAA data. gpx file
UK Met Office forecast areas made manually with tracks and waypoints. gpx file
Some offshore wind farm areas. A kml file converted with gpsbabel. gpx file
Another kmz file converted to a gpx file. This very large file describes all the Exclusive Economical Zones world wide. Much more on this site www.justmagic.com/GM-GE.html
Note that far from all kmz/kml are suitable for conversion. gpx file