Windows only: HeatMapper is a really novel way to map out the signal strength of wireless networks. Armed with a wireless-enabled laptop you can map out signal strength, security issues and more.
First the one downside: There is no basic floor plan/map creation tool built into HeatMapper so you'll need to import your own. HeatMapper will accept floor plans, campus maps or other layout diagrams in PNG, BMP, JPEG, and GIF. The image doesn't need to be perfect so dropping in a picture you scanned off the building fire-exit diagram or sketched up really quickly in Microsoft Paint should do. You can forgo importing a map and just use the default grid, but it won't have much value for anyone without easy to visualise markers and boundaries.
Once you have a map imported into HeatMapper, you simply walk slowly around the area the map represents. When you are at a location on the map you want to survey, left click on it on the map. The more surveys you perform the more accurate and detailed your heat map will be. When you are done with your walk around the survey area, you'll have a map, like the one seen here, that shows the signal strength of various wireless access points.
You can interact with the map by selecting different wireless hot spots to see just the coverage they provide or see all the coverage simultaneously. You can also look at a list of the individual access points and see data about them like the SSID, channel and type of wireless technology, what security protocols they are using, the MAC address, the signal strength, and the time the access point was discovered. With a little walking and mouse clicking you'll have a detailed map of the wireless networks in your home, neighbourhood, or workplace. HeatMapper is freeware, Windows only.