I'll have to disagree with Pugglewuggle to a certain extent.
Netstumbler is a great program, I have it and use it quite a bit.
However it doesn't have the mapping or reporting tools that packages such as AirMagnet, Berkley, Fluke, Ekahau, etc.
If this is a one-time project on your own network, then netstumbler is just fine.
But, if this is for a client and you plan on doing more surveys, then invest in the right software.
