WiFiSpy is a simple Mac application that sits on your Menu Bar and displays the current AirPort link speed. This program might prove to be of good use if you're transferring a large file via Wi-Fi and wish to know where you have the best signal strength possible.
You can choose between 3 styles for the application's status bar icon and configure the program to automatically launch at system login. Once the AirPort is on and connected to a network, the utility starts displaying the current speed in megabits per second.
In addition, the main menu of the application also shows you the name of the network you're connected to via Wi-Fi.
Other than that, not much more can be said about this program. It does exactly what's being said on the homepage of the developer, without slowing down the performance of other running applications.
In conclusion, you should give this Mac utility a try if you're looking for an easy and quick way to monitor your current AirPort link speed. The application is very simple to configure, provides you with accurate information, and is free.