Mapnik is a toolkit for developing GIS applications. At the core is a C++ shared library providing algorithms/patterns for spatial data access and visualization. Essentially a collection of geographic objects (map, layer, datasource, feature, and geometry), the library doesn't rely on "windowing systems" and can be deployed in any server environment. It is intended to play fair in a multi-threaded environment and is aimed primarily, but not exclusively, at Web-based development. High-level Python bindings (boost.python) facilitate rapid application development, targeting zope3, django, etc.
Roadnav is an in-car navigation system capable of running on a variety of operating systems, including Linux, Windows, and Mac OS X. It can obtain a car's present location from a GPS unit, plot street maps of the area, and provide verbal turn by turn directions to any location in the USA. It uses the free TIGER/Line files from the US Census Bureau to build the maps, along with the GNIS state and topical gazetteer data from the USGS to identify locations.