I have used both the NASA and the Silva (Nexus now I think) and found them both excellent. Both have NMEA input and I also daisychained in the DSC VHF with a Garmin GPS72, and a lappie.
Most GPS functions can be displayed, and a highway map for Cross track error.
Obviously they dont show chart detail.
I think the first reply was not about additional laptop computer display
An extra display for your laptop is not an simple thing. Most computer displays have not enough backlight to be of any use outdoors. You are looking for a sunlight readable display. Those can be quite expensive. Also the resolution of those display is typically a lot lower that that of modern laptop screens.
We use splash proof screen bought on ebay. Runs straight from 12v plug on deck. Extended VAG and USB cables from chart table. Generaly run only the screen on deck but will run from laptop below too. For the laptop we generally use an old Dell latitude 266 with PCplotter software as shown in the below deck photo. So far has survived two seasons including sail from Cumbria to La Baule in South Brittany and back last year. Has survived several gales including 44 knots of wind in the channel last summer on the way home. Would not be with out it now.
Costs
Screen £35, Dell comp £10, was being thrown out as battery was US. cables from old computers free. Software PC plotter plus navionics charts http://i266.photobucket.com/albums/ii272/crewcharlotte/IMG_1943.jpg http://i266.photobucket.com/albums/ii272/crewcharlotte/IMG_2087.jpg