Another useful free tool I love is [[xref:https://market.android.com/details?id=com.farproc.wifi.analyzer|WiFi Analyzer|Android Market :: Wifi Analyzer]]. This app grabs the Android phone's WiFi service, looks for access points, and displays their details, including signal strength, channels in use, AP SSIDs, encryption systems used, and Mac addresses.
Five views are available including Channel Graph (plots channels in use against signal strength), Time Graph (channel signal strength against time), Channel Rating (a simple bar graph of channel use), AP List, and a Signal Meter that shows current signal strength as a conventional needle and dial display. I'd like to see the ability to capture snapshots and email data, but for free, there's really nothing to complain about. WiFi Analyzer also gets a rating of 5 out of 5!