Network Computing turns in a long feature looking at tools that help troubleshoot and monitor Wi-Fi networks' health by examining the radio frequency space they operate in: The monitoring space is very hot; I'm asked all the time to be briefed on the latest product or product revision. I've demurred because Wi-Fi Networking News lacks an RF testing environment. The folks at Network Computing show how to examine the field. They look at a dozen products from several manufacturers, only some of which directly overlap by function.
The reviews sort products into spectrum analyzers, calibration tools (just a single entry), wireless protocol analyzers, site surveying tools, and performance and security evaluators. Spectrum analyzers look at the RF patterns and are typically intended for more sophisticated users and uses; protocol analyzers deconstruct data into known patterns to see how the airspace is being used by data devices. A spectrum tool can show if a microwave oven is disrupting your network or an errant cordless phone, while a protocol tool will let you see how 15 networks on the same channel in a small space are producing throughput degradation.
A big caveat in the protocol analysis category is how well the products can decrypt packets when you have the network keys, and how they deal with authentication and dynamically generated Wi-Fi encryption keys.
To cap off the article, there's a sidebar on free tools that can perform a subset of the functions covered in the feature.