I've used OziExplorer with USGS (US Geological Survey) topographic maps in Tiff/DRG format while at work (I'm an archaeologist). The software and map loading takes a little effort, but nothing else comes close. Also, many maps from OziExplorer are free and there are links to user produced maps on their website.
USGS topo maps of the entire United States at 1:24,000 scale or DRG files (Digital Raster Graphics) are freely available to download at the Libre Map Project. The level of detail at this scale is the best for off-road adventures (it shows individual buildings and such); however, it takes quite a few maps if you are covering a large area.
USGS maps at the Libre Map Project
Also consider Fugawi:
Fugawi.com
If you want to get really technical (and expensive), GIS software, such as ArcView can display any type of map or digital image available with a little work. This is technical software and not really set up for carputer use, but it can be done.