A hint last year mentioned Wi-Fi Diagnostics, an application hidden in /System/Library/CoreServices. It turns out that you can also access this by Option-clicking the AirPort menu in your menubar and choosing Open Wi-Fi Diagnostics.
[kirkmc adds: This was available under Lion, but wasn't in the original hint, so it's worth posting.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120905082310287