Update 23/04/2014: The installation procedure described below no longer works on Mavericks. Instead you must download the Hardware IO Tools from the Apple Developer portal (search for Hardware IO Tools and download the latest .dmg available). From that .dmg, you’ll be able to install the preference pane.
Update 25/09/2012: Network Link Conditioner also available in iOS6 on devices.
Since the release of OSX Lion, the Apple Developer Tools have included a nifty tool to help developers simulate different network conditions. This is particularly useful for iOS developers who want to test and debug apps via the simulator and pretend they have an Edge/ or 3G-like network connection. To install the Network Link Conditioner, go to the Network Link Conditioner folder located under “/Developer/Applications/Utilities/” and install the preference pane (reminder: You must have OS Lion or above installed). Once installed, you’ll see this :
Select whichever profile suits your needs most (or create one) and then turn it ON :
Of course, don’t forget to turn it OFF when you’re done!
Update : Added full path of location. Thanks @vinzius.