There are two major categories of the free computer mapping programs:
Desktop mapping software
This software is downloaded and installed locally and runs on your Windows based personal computer. Examples below include ArcGIS Explorer from Esri, and Google Earth from Google. Other examples not mentioned include a free Esri mapping application and Business Analyst Online, both are apps that run on Iphones and Ipads. ArcGIS Explorer can work with data on your local hard drive in many formats, and Google Earth will work with local data but in fewer formats. Both benefit from an Internet connection but can run without one.
Web browser based mapping software
These mapping software packages run entirely within a common web browser such as Internet Explorer, Firefox, or Chrome, etc. You need an Internet connection to run them. There is typically no software to install locally, with the exception that you sometimes have to install add on programs for your browser. An example would be mapping applications that require additional Adobe that are not part of your browser by default, like a flash viewer or flex, or Microsoft Silverlight. Normally the mapping application notifies you of what you need and where to download it.
The other major distinction between web browser based mapping programs is some are general mapping programs that have some flexibility to add additional map layers (for instance the web version of ArcGIS Explorer). The other category are those that are dedicated to a single purpose such as providing property data, wildlife data, or water rights, etc. These are built by government agencies, universities, and private companies. In most cases all the data used in any web browser mapping program resides on servers elsewhere in the world and not on your local computer or smartphone device. Another way to say that is the data resides "on the cloud", or is a "cloud based service"
ArcGIS Explorer Desktop
ArcGIS Explorer Desktop is a free GIS viewer that gives you an easy way to explore, visualize, and share GIS information. ArcGIS Explorer adds value to any GIS because it helps you deliver your authoritative data to a broad audience.
With ArcGIS Explorer, you can:
Access ready-to-use ArcGIS Online basemaps and layers.
Fuse your local data with map services to create custom maps.
Add photos, reports, videos, and other information to your maps.
Perform spatial analysis (e.g., visibility, modeling, proximity search
Download and Install on your PC: http://www.esri.com/software/arcgis/explorer/download.html
Tutorial for ArcGISExplorer - ArcGIS Explorer Quick Start Tutorial. Check out the description at http://training.esri.com/acb2000/showdetl.cfm?DID=6&Product_ID=961
Introduction to ArcGIS Explorer - Introduction to ArcGIS Explorer Online. Check out the description at http://training.esri.com/acb2000/showdetl.cfm?DID=6&Product_ID=973
Google Earth
Zoom from space to street level — tour the world
Find maps, driving directions, hotels, restaurants, and more
To Download Google Earth 6
http://www.google.com/earth/index.html
ArcGIS Explorer Online With ArcGIS Explorer Online, you can easily explore, visualize, and share GIS information. Use it to directly access basemaps and other content. You can even use it to create an interactive slideshow. ArcGIS Explorer Online is a free online application that has a similar look and feel to the downloadable ArcGIS Explorer Desktop. It also has some of the same capabilities. ArcGIS Explorer runs in a browser and is built using Microsoft Silverlight.
http://explorer.arcgis.com/
Montana Department of Commerce Broadband Mapping Project
http://www.mtbroadband.org
Montana Fish Wildlife and Parks CAPS Project
http://fwp.mt.gov/wildthings/conservationInAction/crucialAreas.html