Is anyone aware of a GIS application that an ordinary person with no GIS expertise could readily download and use? By way of analogy, the average Joe cannot download and immediately benefit from AutoCAD - it costs too much and the learning curve is steep. However, many people benefit from Google SketchUp, which is free, easy to learn, and powerful.
Let me throw out a hypothetical example: I own a small lawn care company. I want to maintain a geographic database of my clients about town. This way I could quickly print a map of clients whose lawns are due for service this week, and perhaps even generate the best route and sequence for servicing them. (CMS+LBS)
Another hypothetical example: I manage building maintenance for an office park. I want to log service requests by location, type, and severity; and generate a map of hot spots. (cluster analysis)
Wrap-up: Thanks for all the great suggestions. It looks like the offerings out there generally fall in one of three categories: "ArcGIS alternative," "ArcGIS lite," or "web-based mapping." The Google offerings are most promising in terms of accessibility, price, and ease of use. Google Earth, Google Maps (esp. My Maps), and Google Fusion Tables are all exciting.