Dirk große Osterhues

Aside from all those OS X wifi scanners originating from the Linux world, OS X comes with its own basic tool called airport. For quick access do the following:

sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/bin/airport

Now, example usages can be:

airport en1 scan

or

airport en1 sniff 6