Navigation software usually costs a pretty penny. Reasons include that it’s a specialized and limited market, and that developing high quality software that will support an ongoing business is expensive. If you have a tight budget and can do without some of the features and polish of the impressive commercial products out there, SeaClear deserves a good look. Great news for Linux users is that SeaClear is said to work just fine with the Wine Windows emulation library.
SeaClear Charting Software Is Free SeaClear II is, as far as I can find, the only fully functioning free electronic navigation chart plotter. SeaClear is PC based, interfaces with GPS, and uses raster charts (such as the free NOAA raster charts) and user-scanned images of paper charts. It’s available for download at no charge at http://www.sping.com/seaclear/.
Also worth checking out is FreeNav, a nice navigation-software-focused blog. A bunch of good articles there.