Roadnav is an open source street navigation solution capable of running on a 
variety of operating systems. It can obtain your position from a GPS unit, 
plot a map of your area, and provide directions to locations in the USA. It 
can also verbalize directions using Microsoft SAPI 5.1, Festival, flite, and 
OS X's built in text to speech engine.

Roadnav uses the free TIGER/Line (Topologically Integrated Geographic 
Encoding and Referencing) 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. It has experimental support for scripting, LCDproc, importing 
OpenStreetMap data, and importing GPX waypoints and tracks. 

WWW: http://roadnav.sourceforge.net/