Wi-Spy is a tiny spectrum analyser the size of a USB flash drive. It measures the 2.4 GHz band used by b/g WiFi and various other things like cordless phones, bluetooth, and microwave ovens. If you are having problems with your wireless network then Wi-Spy is very useful for working out what's going on and what you can do about it. For example, here's what it shows in my living room:
The numbers at the bottom are the WiFi channel numbers. There are two WiFi networks visible; mine is on the left, centred on channel 1, and my neighbours is in the middle on channel 6. On the right hand side there's something that's not WiFi; I'm not sure what it is but it's quite powerful and it would make WiFi on channels 11 to 13 very problematic.
I bought myself a Wi-Spy because my network on channel 11 stopped working. It would have been possible to fix this by trial-and-error, but having the Wi-Spy display makes it much quicker and easier.
The only disadvantage is that it's not cheap: it costs about £160 for something that you might only use for a few minutes. So as a service to other people who might be suffering WiFi problems and to perhaps recoup some of my outlay, I'm making my Wi-Spy available for hire for £25 a time.
If you would like to borrow it, get in touch by email. You can send your £25 by bank transfer, cheque or paypal. I'll send the Wi-Spy by post and you can keep it for a couple of days, or maybe a bit longer if your problem is hard to track down, and then return it.
There is more information about Wi-Spy on the manufacturer's website; it includes examples showing the waveforms that you might see for various common types of interference.
Note that this is the basic model; they have other versions with different frequency ranges etc.
The device is most easily used plugged in to a laptop; it has software for Windows, Linux and Mac OS. If you are in the Edinburgh area I might even be able to lend you my Eee to use it with.