This is the free chartplotter software I have been using for several years now. Perhaps not as fully featured as commercial products, but it works with most raster charts. NASA Marine supply it with their AIS engine to display AIS targets on your laptop. A nice feature of Seaclear II is the multiplexing capability. It can handle three incoming NMEA datastreams (either through serial or USB ports) with different speeds and integrate them into one NMEA datastream for on-screen use or for output (through another serial or USB port) to external NMEA receiving equipment. For example: my Raymarine equipment supplies NMEA at 4800 bps, my GPS mouse at 9600 bps and my NASA AIS receiver at 38,400 bps. Seaclear eats it all.
http://www.sping.com/seaclear/
Use it at your own risk.