About the best 'free' viewer that will load RNC (Raster charts) and ENC (Electronic charts) is Rose Point Navigation Systems - Coastal Explorer
. Their free viewer has a time limit on GPS, but provides chart viewing and route planning with no time limits. Once you have all of the maps in the list, you can right click on the map and select between raster or electronic, or even both (so as you zoom in, it picks which has better detail).
has one as well, but it only does RNCs, and charts are viewed independently, so it's not as easy to navigate across charts.