FJ Cruisers of Northern California
Now the main app is free for Android and iOS, however the Forest Service maps run about 5 bucks per Forest (i.e. Eldorado is one map, Plumas is another, etc) However if you create a login for the application any maps you buy can be downloaded to other devices. I have tested a map I bought on my Nexus 7 was able to be downloaded at no additional cost on my iPAD and on my Nexus 5.
The quality of the maps seem pretty good and the are easy to read and show the current OHV trails, camp grounds The other nice feature is that they will work with the GPS of the device and show you where you are on the map you are looking at and you can also drop pins for different spots and add notes them.
I have not done a trail run with these yet, but the fact that you can buy 1 map and use it on multiple devices cross-platform makes this worth checking out and it seems to be the easiest way to get FS maps next to ordering physical maps
The Screenshot below is from my Nexus 7
borat voice - Veeeri Naaice!!