Freeware February: MacStumbler
MacStumbler is only at version 0.75b, but for an application that hasn't hit 1.0 yet, it rocks. A very simple app that uses your Airport card (sorry, non-Apple Wifi cards don't work with it)
it can scan your immediate area for 802.11b and 802.11g networks and display a variety of information about them.
For all you Wardrivers out there (and you know who you are) MacStumbler can interface with a GPS unit to produce a map of the wireless networks in your area.
The only downside about this app is that development seems to have come to a standstill. Perhaps being selected as a Freeware February pick will ignite the developer to get back to work on it.
Update: Jack Beckman tells me, via the comments, that MacStumbler is now
iStumbler, which accounts for the lack of updates. iStumbler is on release 91.