Network Link Conditioner in Lion
Test apps and websites under various network conditions.
In the Finder, take a look in the Utilities folder (Go menu, Utilities – or press Command Shift U). You should see a folder called “Network Link Conditioner”, which contains a preference pane for System Preferences.
I assume that this folder is installed by the (free) Developer Tools, because I didn’t find it on two other Lion-running Macs which didn’t have the Developer Tools installed. Double-click the prefpane file and authenticate to allow it to be installed. You’ll then see the pane in System Preferences.