MetaGeek Wi-Spy DBX spectrum analyzer
You would not believe how much RF there is in the air, especially at the 2.4GHz band 802.11b/g uses. The MetaGeek Wi-Spy DBX spectrum analyzer has been a constant companion that has explained a huge variety of Wi-Fi-related problems, and it's a tiny fraction of the cost of what I used to pay for luggable spectrum analyzers. I carry the DBX version around for 2.4GHz and 5.8GHz vision, and with MetaGeek's Chanalyzer app available for Windows and Mac, I swap it back and forth between my machines. Combine it with MetaGeek's newest offering, Eye P.A., and you get superb wireless packet analysis.