Though a map or atlas may not technically fit in the "gear" category, it is something I could not have done my cross country trips without so I'm including them. Folding maps are great for regional or in-state rides but if you are going to be covering real ground, a good atlas is a must. Google maps is also a great tool. You can tailor directions to your own needs and print them with the related maps, scaled to your liking. This would be a bit daunting to do for a long trip but for smaller ones, I use it all the time.
The atlas you choose can make or break your trip. The bigger, the more complete and detailed it will be. You can see a comparison below of the small and larger atlas.
This is Delaware (and Maryland) on the small atlas. You can see it is not nearly as detailed as the map below. Putting two states on a page isn't helping anything either.
From the larger atlas, one state spread across two pages. Tons of detail, including an inset of a major city.
Here is a closer shot. This atlas had markers for camp sites, state parks, national parks/forests, hospitals and points of interest. This was a must for me. My travel plans are pretty thorough but things happen that you cannot plan for. Being able to look and find alternate campsites was a big plus.
Here is a shot of my most recently acquired Alabama map. I make all kinds of highlights, additions, marks and numbers when a map is less detailed.
On the road to Canada, at the end of each day I would highlight the route we had taken and plan the next day. I know some folks who use a GPS on long trips. I know this wouldn't work for me, i like having the big picture and being easily able to go back and forth.