Where does the iPhone Simulator store its data?

There is another (faster?) way to find where your app data is without Terminal:

1. Launch the app in the simulator
2. Open Activity Monitor
3. Find the name of your app in the CPU tab
4. Double-click it and open the "Open Files and Ports"

For Xcode6+/iOS8+

~/Library/Developer/CoreSimulator/Devices/[DeviceID]/data/Containers/Data/Application/[AppID]/

Accepted answer is correct for SDK 3.2 - SDK 4 replaces the /User folder in that path with a number for each of the legacy iPhone OS/iOS versions it can simulate, so the path becomes:

~/Library/Application Support/iPhone Simulator/[OS version]/Applications/[appGUID]/

if you have the previous SDK installed alongside, its 3.1.x simulator will continue saving its data in:

~/Library/Application Support/iPhone Simulator/User/Applications/[appGUID]/

Easiest way ever.

1. Catch a Breakpoint somewhere. (or Pause program execution (tap on pause in debug area) as Najdan Tomić mentioned on the comments)

2. Enter po NSHomeDirectory() in console window

Result:

(lldb) po NSHomeDirectory() /Users/usernam/Library/Developer/CoreSimulator/Devices/4734F8C7-B90F-4566-8E89-5060505E387F/data/Containers/Data/Application/395818BB-6D0F-499F-AAFE-068A783D9753