What are good resources to use to find dirt roads, fire roads, gravel roads, or disused paved roads for planning a gravel grinder or ultracross style ride.
Any particular atlases, online mapping resources, government departments to check?
Good quality road maps will differentiate between paved and unpaved roads. I really like my Gazetteer State Maps
For those still interested, there's a pretty solid map site available now covering the US. Not everything is on there, and you can add more gravel roads as you find them.
I found this map of Pennsylvania with dirt roads overlaid a while back. Not aware of similar projects but I imagine/hope they are out there.
Without knowing your location I can't give specifics. If you check for a local or regional mountain bike assoc. they should have some trail guides. In the northeast we have the new england mountain bike assoc. Most of these groups have websites with trail reviews. Not all of the trails are singletrack or downhill many are cartroads and firebreaks. Many trails are accessed via fireroads with descriptions and difficulty ratings. I use MTBR.com they have a state by state listings and reviews from users. Some will have maps or links to get them. Some have gps coordinates to locate key areas.