How do I run the iPad simulator to test a website on my mac
Install XCode, then the simulator is located at /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app
Once it's running, go to Hardware -> Device, then pick the iOS version and device.  It will boot up, then you can start Safari.

Update: XCode.app is now a package rather than a folder. In Finder, option/right click on XCode.app and select 'Show Package Contents', then follow the path down from there.