There are a number of GPS and navi apps available for Android, so it just depends on what suits your task. Some come with complete map sets for the areas you wish to use. I'm a fan of CoPIlot which you can download with the complete US or North American road database. There is a free version, but the paid version is worth it, at least for on-road navigation. Navigon is another; more expensive but with a slightly better road network. For off-road, some of the others mentioned already would work very well. The cool thing with using an Android is that you can install as many of these programs as you want; limited only by space on your SD card!
K13 Tiger Explorer XC, K8-DL1000, K7-KLR650