Xastir is an amateur radio mapping and tracking application which supports hundreds of map formats, including both local and online maps, and several operating systems.
Xastir can be coupled with a GPS & TNC/Radio combination to create a mobile tracker, a GPS to create a moving map system, or can watch other mobile trackers via TNC/radio or internet connections. It can also be used to monitor severe weather alerts in a graphical format.
Check out the screenshots at http://www.xastir.org and some severe weather screenshots at http://wxsvr.net