You can toggle individual base stations in a multiple-unit network to get a sense of how well each base station covers the map. If you have areas of poor reception, this information can be used to show where you might move a router to improve coverage (and then, of course, follow up with a new bit of NetSpot testing). You can also determine whether you need to add a base station.
Each kind of visualization can be exported as a PDF that you can print out or carry around on your various digital devices. Base stations checked in the detected list are included in a chart, as well as on the colorized map.
My old house has lathe-and-plaster walls, thick joists, and basement storage that block Wi-Fi signals. Using NetSpot, I confirmed that my aforementioned three base stations (which are connected by ethernet) are set up in locations that are close to optimal. But it also helped me discover why a laptop upstairs always connected to a base station downstairs: The laptop is used directly above one of the basement base stations, and despite the floor and ceiling in between, that base station provides the strongest signal of any of the three access points.
NetSpot's developers told me that this free version is their first attempt at building Mac OS X wireless site-survey software. A free version will always be available, but they plan to eventually release a paid version with more, and more-advanced, features. That's good news. NetSpot is already an easy tool to use and master for one-time surveys of modest setups. Those with more significant deployments, or networks that are set up and torn down regularly, will surely benefit from whatever the developers can add to this nifty toolkit.
Glenn Fleishman, a senior contributor at Macworld, is the author of Take Control of Your 802.11n AirPort Network, recently updated for Lion.