KisMAC allows users to detect and monitor wireless connections. The application uses AirPort (by default) to scan for SSIDs (service set identifier) but also supports many USB wireless devices. KisMAC may be used for testing out the security of your own Wi-Fi connection or cracking its password (WEP and WPA).
The utility provides multiple display modes for the connections located in your vicinity: a global view (dynamic map that supports zoom in and out), connection details, traffic and network. With KisMAC you will be able to simulate attacks (in order to test the strength of your connection) or crack your Wi-Fi connection with one of the three types of attacks: wordlist, weak scheduling, bruteforce.
What differentiates KisMAC among other net stumblers (like AirRadar, iStumbler) is the ability to use both active (monitor) and passive scanning modes (passive scanners can not be detected).
I found KisMAC quite intuitive to use for simple tasks like detecting Wi-Fi connections and locating them; for more advanced tasks (cracking purposes), I suggest you read the online tutorials and guides.