When I started designing some very early mesh networks (DARPA SensIT project) I had to fork over something like $5,000 for a refurbished spectrum analyzer. Today that same capability can be found in the Wi-Spy DBx USB spectrum analyzer and Chanalyzer software from MetaGeek. Kudos to these geeks for making their tool available for Windows, Mac, and Linux. Although variations exist between the versions, they all provide me with a clear and concise look at the garbage in the RF range known as the ISM band. Most importantly it has helped me track down all sorts of goofy things -- Mr. Microphone like devices, leaking microwave ovens, collections of high pressure sodium lights, and other truly odd sources of radio interference living in and around the 2.4GHz range. This isn’t a free tool but Metageek has collected some really good free or inexpensive WiFi tools on their website in addition to their commercial products that pay the bills.
Price: $599 MSRP for Chanalyzer 4 with the Wi-Spy DBx radio
Compatibility: Mac OS X 10.6 or later