There are several free sources for additional layers in shapefile format that will work with ArcExplorer and can be added as themes to any map you are working on. Check the sources on the next page and also click here for my archive of free map layers.
For this example I have chosen to download the National Parks map layer from BTS (Bureau of Transportation Statistics). Click on the link below, "BTS 'background' map layers." The link will take you to this page:
Scroll down to National Parks and download it to your c:/mapcruzin_maps/downloads/ directory.
Once downloaded you can add it to your project using the previous steps for adding a theme. Add other layers in a similar manner.
One thing to note. As you learned earlier, ArcExplorer will open themes that are in zip format for you. When it does this it actually unzips the file and adds the corresponding shapefiles to your directory.
If you add the zipped files as themes to a project that already contains some themes, you may find that ArcExplorer will make you create a new project that only includes the most recent added theme. To get around this, you can use the Theme --> Add Theme menu to add the previous themes that are seemingly "lost." Make sure that you add the file(s) with the "shp" file extension rather than those with the "zip" extension.
Click here to download any of the following BTS "background" map layers (As of 091506 the above was still available, but recently many things have been removed from this website. If you can no longer find this data, please email me at mike@learn2map.com and I will make our archived copies of this data available to you.):
Points
Public-Use Airports
Amtrak Stations
Automatic Traffic Recorder Stations
National Populated Places
U.S. Army Corps of Engineers Ports
Intermodal Terminal Facilities
Weigh in Motion Stations
Polyline
Hazardous Material Routes
Highway Performance Monitoring System
Railway Network (1:2,000,000 base scale)
United States (line)
United States (node)
Public Use Airport Runways
Fixed-Guideway Transit Facilities
Links
Stations
U.S. Army Corps of Engineers Navigable Waterway Network
United States (line)
United States (node)
Polygons
Bureau of the Census Urbanized Area Boundaries (1:100,000 base scale)
The 109th Congressional Districts Boundaries (1:100,000 base scale)
U.S. County Boundaries (1:100,000 base scale)
Hydrographic Features
United States (line) (Data Dictionary)
United States (polygon/arc)
U.S. Military Installations
Metropolitan Planning Organization
National Metropolitan Statistical Areas (1:100,000 base scale)
Non Attainment Areas
Carbon Monoxide
Lead (Data Dictionary)
Nitrogen Dioxide
Ozone
Particulate Matter 10
Particulate Matter 2.5
Sulfur Oxides
National Park System Boundary Dataset
U.S. State Boundaries
Downloading National Atlas Map Layers
Click here to download the following National Atlas map layers. Before you download, you can read a detailed description of the layer(s) that interest you. Be sure to click on the "shapefile" to download. Note that the atlas is being "built" on an ongoing basis. New map layers are added continually so check the National Atlas page often if you are interested in this material.
Reimagination - Reimagining, exploring and celebrating the changes in infrastructure, politics and culture that will help us live in harmony with each other and the earth now and in the future.
There are several free sources for additional layers in shapefile format that will work with ArcExplorer and can be added as themes to any map you are working on. Check the sources 
