Did you know that OS X has a built in WiFi survey tool? Nope, neither did I, but here it is

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport

This won’t be in your path, so to make things easier you could add the following to your ~/.profile

% alias airport=/System/Library/PrivateFrameworks/Apple80211.framework/Versions/A/Resources/airport

Here is an example of the output.

% airport -s
          SSID BSSID             RSSI CHANNEL HT CC SECURITY (auth/unicast/group)
        fibble 00:26:de:ad:be:ef -59  9       Y  -- WEP
       flobble 00:60:ca:fe:ba:be -66  6       N  AU WPA(PSK/TKIP,AES/TKIP) WPA2(PSK/TKIP,AES/TKIP)
         wibble 94:44:51:51:51:51 -62  6,+1    Y  -- WPA(PSK/AES,TKIP/TKIP) WPA2(PSK/AES,TKIP/TKIP)
         wobble 00:21:af:af:af:af -60  3       N  US WPA(PSK/TKIP/TKIP)
          ftang 00:18:3b:00:b1:15 -73  11      N  -- NONE

For the nitpickers, this data is also available in WIFi icon on the menu, with the important distinction that you can only find out the channel and signal strength if you associate with the AP. Using this tool you can easily do a quick WiFi survey to pick a good channel to avoid side lobe interference (hat tip to Matt Ryall).