"Wi-Spy is cheap, small, and easy enough to use that it can be a major step towards compensating or eliminating interference with your WLAN." -Wi-Fi Planet
inSSIDer is one of the web's greatest tools to track Wi-Fi signal strength! Like most other professional Wi-Fi troubleshooting software, it can't display all radio interference. Wi-Spy 2.4i shows what the 802.11b/g wireless environment looks like in real-time so you can avoid those pesky microwaves, wireless video cameras, or cordless phones which a standard wireless card can't display.
Use Wi-Spy to choose the clearest channel for a WLAN. Choosing a good channel will help to avoid call backs, or endless amounts of troubleshooting, At this price, you won't find a better solution to your wireless woes.
MetaGeek has an excellent library of Wi-Spy recordings. Browse through all of the different interference profiles to understand your own wireless environment.
Intro to Wi-Spy 2.4x from trent on Vimeo.