I'm surprised OpenCPN has not had any mentions here - it's a free of charge, free software navigation system that is under development but apparently fully usable. Features of the most recent version, 1.3.0, include CM93 vector charts, AIS support and runs on Windows, Mac and Linux.
http://sourceforge.net/projects/opencpn - OpenCPN project page at Sourceforge
http://bigdumboat.com/cpnindex.html - Lead developer's homepage
Enjoy!!
http://sourceforge.net/projects/opencpn - OpenCPN project page at Sourceforge
http://bigdumboat.com/cpnindex.html - Lead developer's homepage
Enjoy!!