If you have an android phone and work in IT support this is an amazingly handy tool to have installed, and it is free.
I found it useful at school trying to track down a rogue access point when we had only 6 to manage. Now I use it almost everyday in my new job where we have over 600 AP's. To install just search for Wifi analyzer on the android market and it is from farproc.
I have found it to be as reliable and accurate as more expensive paid for products like the Fluke Etherscope (a very good wireless analyzer) that is rather expensive. It is also a very small program (less than 0.4mb) so won't take up a lot of space.
It has a channel graph that will also show you signal strength which could be handy if you have any problems with access points crossing channels:
It also has a time graph that you can filter so you can walk around testing the signal strength in various locations. You can also list all the AP's in range and see detailed information about them like mac address, IP, SSID, etc:
There is also a signal meter. Quite often we have wireless access points in the roof space of buildings to prevent tampering or because the building owners do not want them spoiling the walls. The signal meter is handy for finding these access points if you haven't been to a site before or if you have forgotten where it is. It can also be setup to play sounds when you are getting closer:
In summary Wifi analyzer is a very handy piece of kit to have in your pocket if you are in IT, data or network support and you cannot argue with the price.